spec.txt

---
title: Markua Spec
author: '*This formal specification of Markua was developed at [Leanpub](https://leanpub.com), is written by Peter Armstrong, is based on the CommonMark Spec (version 0.31.2) by John MacFarlane, and is licensed under the Creative Commons license CC-BY-SA 4.0.*<br/><br/><hr/><br/>NOTE: If you are a Leanpub author, there are two different versions of Markua used on Leanpub. <strong>The default version of Markua for books on Leanpub is Markua 0.30, which is documented in this specification, and which is currently in beta on Leanpub.</strong> The default version of Markua for courses on Leanpub is still Markua 0.10, which is documented in [The Markua Manual](https://leanpub.com/markua/read). (You can still use Markua 0.10 and Leanpub Flavoured Markdown for books on Leanpub, of course.)<br/><br/><hr/><br/>'
version: '0.31.36'
date: '2024-04-30'
license: '[CC-BY-SA 4.0](https://creativecommons.org/licenses/by-sa/4.0/)'
...

# Introduction


<div class="extension">

## The magical typewriter (M)

Imagine you owned a magical typewriter.

When you used this magical typewriter, you wrote with fewer distractions. You
didn't just write faster, you wrote better.

With your magical typewriter, you never worried about layout. The book formatted
itself.

You could hit a key on your magical typewriter to create an ebook from your
manuscript with one click.

All ebook formats would be created, and they'd all look good. You'd have PDF for
computers, and EPUB for everywhere else. The book would look great on phones.

With your magical typewriter, you could publish your book before it was even
done, and get feedback from readers all over the world. You could automatically
share book updates with them. You would press one key on your magical typewriter
to publish a new version, and all your readers would have it instantly.

With your magical typewriter, you could easily compare your current manuscript
to any other version of your manuscript that had ever existed.

If you decided to make a print book, you could press a key on your magical
typewriter to get a print-ready PDF. All you would need to do is add a cover.

If you wanted to work with a designer, you could press a different key to
generate InDesign. They could then use this as a starting point for producing a
beautiful print book.

Or, if you wanted to work with a publisher, you could press a different key to
get a Word document.

Your magical typewriter could even transform your completed book manuscript into
a course that anyone in the world could take. All you'd need to do is to
add some quizzes and exercises and then press a key for your magical typewriter
to publish an online course for you. The quizzes and exercises would mark
themselves, and students would get certificates based on how well they did.

With your magical typewriter, you'd only have to do one thing:

Write.

Wouldn't it be great if such a magical typewriter existed?

It does. At [Leanpub](https://leanpub.com), we're building it.

But there's one requirement for this magical typewriter to exist: a simple,
coherent, open, free, plain text format for a book or course manuscript.

This simple format will be the basis for the magical typewriter.

This simple format is called Markua.

This is its specification.

## Markua: Markdown for books and courses (M)

Markua, pronounced "mar-coo-ah", is Markdown for books and courses.

Markua maps Markdown syntax to book concepts, and then adds some new syntax and
concepts of its own.

Markua documents can be automatically transformed into every popular type of
ebook format. The computer programs which do this transformation are called
Markua Processors. These programs understand both Markua syntax and how to
generate the various output formats. An example Markua Processor is Leanpub:
Leanpub can output PDF, EPUB and HTML from the same Markua document, and can
even output print-ready PDFs and InDesign files from them as well.

Markua has been developed with extensive real-world testing and feedback.
Markua has been used by Leanpub authors for years, both to create books and
online courses.

Markua was started at Leanpub in 2014, and benefited from the years of lessons
that Leanpub had learned from Leanpub Flavoured Markdown (LFM). Markua is the
successor to LFM. We have been iterating on our Markua support for many years.

</div>



<div class="extension">

## Markua's five key additions to Markdown (M)

Markua's five main additions to Markdown are the following:

1. The mapping of **Markdown headings (h1, h2, h3, etc.) to book structures**
   (chapters, sections, sub-sections, etc.), which provides the ability for
   Markua Processors like Leanpub to produce an ebook from a Markua manuscript
   with one click.
2. The **unified resource syntax**, which lets Markua handle audio, code,
   iframe, math, verbatim and video resources in the same way that it does
   images and tables, and which supports inline, local and web resource
   locations.
3. The **attribute list syntax**, which lets Markua define properties for
   resources using the same syntax that is used for other document elements,
   such as part headings and index entries.
4. A comprehensive definition of **document settings**, so that authors can use
   Markua to produce documents which are styled via global switches, rather than
   by tedious micromanagement.
5. The mapping of book structures to **online courses**, and the creation of a
   plain-text microformat for course **quizzes and exercises**, which supports
   automated marking and automated production of everything which is required
   for an online course.

If you have written something (say blog posts or lecture notes) in Markdown, you
can use a Markua Processor, such as Leanpub, to turn them into an ebook or
course with one click. Then, as you go down the path of enhancing the manuscript
and adding things which only make sense in books or courses, this process will
feel like decorating, not converting.

The goal is for writers who are familiar with Markdown to feel that with Markua,
Markdown just grew an understanding of book and course concepts.

</div>



<div class="extension">

## Markua's one key subtraction from Markdown: raw HTML is removed (M)

Markua also has one key subtraction from Markdown: support for raw HTML, either
as inlines or blocks, is completely removed.

Markdown and CommonMark support raw HTML. Markua does not support raw HTML.

Instead, in Markua **all raw HTML elements are removed**.

This is true both for raw HTML which is inserted as leaf blocks, and raw HTML
which is inserted as inlines. Neither of these are supported by Markua.

In both cases, **raw HTML must be completely removed by the Markua Processor.**

As a side effect of this, since HTML comments are HTML, authors can use HTML
comments (`<!-- a comment -->`) as comments in Markua: since these comments are
HTML, they will be completely removed from the output (along with any other
raw HTML tags).

Markua and Markdown have different use cases. Markdown is a better way to write
HTML; Markua is a better way to write a book or course.

HTML is just one possible output format of Markua, and other possible Markua
output formats (such as LaTeX and PDF) are not based on HTML. If HTML blocks
were supported, a Markua Processor would have to support parsing and
meaningfully outputting all of HTML syntax as well as all of Markua syntax.

Since Markdown's only output format target is HTML, it might as well support
HTML blocks: generating HTML from HTML is as simple as passing the HTML through.
From an implementation perspective, Markdown gets inline HTML support for free.
This is not true in the case of Markua.

Note that besides raw HTML, Markua does not remove anything else from Markdown.
(As is discussed later, the Markua spec is written with the CommonMark spec as a
starting point, so that you can verify this for yourself.)

</div>



<div class="extension">

## How to write a novel in Markua (M)

The Markua specification is long. However, if you're an author, the amount you
need to learn to write in Markua is actually very short.

This example shows everything you need to know to write a novel in Markua:

* * *

~~~
# Chapter One

This is a paragraph. You just write.

Blank lines separate paragraphs. This is *italic text*.

This is another paragraph. You can manually wrap your paragraphs
however you want. Single newlines function like single spaces by
default.

* * *

That was a thematic break, which is used as a "scene break" in a novel.

# Chapter Two

This is a paragraph in a new chapter.
~~~

* * *

By the way, everything in the above example is identical in the following
Markdown dialects:

* Markua as specified in this document
* Leanpub Flavoured Markdown as described in the
  [Leanpub Flavoured Markdown (LFM) Manual](https://leanpub.com/lfm/read)
* GitHub Flavored Markdown as specified in the GFM Spec
* CommonMark as specified by John MacFarlane in the CommonMark Spec
* Markdown as originally described by John Gruber

In the simple case, Markua is just a mapping from Markdown concepts to book
concepts. This is deliberate.



## How to write almost any book in Markua (M)

The longer example below shows everything you need to know to write almost any
type of book, such as a computer programming book, in Markua. This example also
serves as a tutorial which explains most of Markua's important syntax in its
content. Some parts of this syntax are just standard Markdown or CommonMark
syntax, and other parts are Markua extensions.

~~~
# Paragraphs

This is a paragraph. You just write.

Blank lines separate paragraphs. This is *italic text* and **this is bold**.

This is another paragraph. You can manually wrap your paragraphs
however you want. Single newlines function like single spaces by
default.

* * *

That was a thematic break, which is used as a "scene break" in a novel.

# Lists

Here's a numbered list (called an "ordered list", even though all lists are
ordered):

1. foo
2. bar
3. baz

Here's a bulleted list (called an "unordered list", for irony):

* foo
* bar
* baz

You can even have definition lists!

term 1
: definition 1a
: definition 1b

term 2
: definition 2

# Tables

You can also use tables, which work best for numeric tabular data
involving a small number of columns containing small numbers:

| Central Bank | Rate      |
|--------------|-----------|
| JPY          | -0.10%    |
| EUR          |  0.00%    |
| USD          |  0.00%    |
| CAD          |  0.25%    |

Note that definition lists are preferred to tables for most use cases,
since typing text in a table quickly gets annoying.

# Headings

Markua supports both of Markdown's heading styles.

The preferred style, called atx headers, has the following meaning in Markua:

```
{class: part}
# Part

This is a paragraph.

# Chapter

This is a paragraph.

## Section

This is a paragraph.

### Sub-section

This is a paragraph.

#### Sub-sub-section

This is a paragraph.

##### Sub-sub-sub-section

This is a paragraph.

###### Sub-sub-sub-sub-section

This is a paragraph.
```

Note the use of three backticks in the above example, to treat the Markua like
inline code (instead of actually like headers).

The other style of headers, called Setext headers, has the following headings:

```
{class: part}
Part
====

This is a paragraph.

Chapter
=======

This is a paragraph.

Section
-------

This is a paragraph.
```

Setext headers look nice, but only if you're only using chapters and sections.
If you want to add sub-sections (or lower), you'll be using atx headers for at
least some of your headers. My advice is to just use atx headers all the time.
(The `{class: part}` attribute list on a chapter header to make a part header
does actually work with Setext headers, but it's really ugly.)

Note that while it is confusing and ugly to mix and match using atx and Setext
headers for chapters and sections in the same document, you can do it. However,
please don't.

Finally, headers and their mapping to document structure concepts are discussed
later in the Document Structure section.

# Images

You can add images like this:

![](mac.jpg)

You can specify the alt text and figure title like this:

![alt text for accessibility](image.png "Figure Title")

An example of the difference between alt text and a figure title is:

![a red apple](mac.jpg "The Original Mac")

You can also set the alt text and/or the figure title in an attribute list:

{alt: "a red apple", title: "The Original Mac"}
![](mac.jpg)

Attribute lists are one of Markua's additions to Markdown, and are discussed
later in this specification.

Finally, you can set an image figure title using the alt text, if the
`alt-title` document setting is set to `all`:

![The Original Mac](mac.jpg)

The default value of `alt-title` is `all`, which means that all resources
can have their title specified in the alt text. There is also a value of
`text`, which means that only text-based resources like code can have their
title specified in the alt text. Finally, there is a value of `none`, which
means that no resources can have their title specified in the alt text. The
`alt-title` document setting is discussed
[in great detail here](#the-alt-title-document-setting-m-).

# Document Settings

Various document settings can be specified at the start of a Markua document to
affect the behavior of the Markua Processor. For example, if you want single
newlines to insert a forced line break in the PDF, EPUB and HTML output formats
(instead of the single space that results by default), set the `soft-breaks`
document setting to `break` instead of the default of `space`:

```
{
soft-breaks: break
}

# Chapter One

This will be on a line
and this will be on the next line.
```

This `soft-breaks` setting is so important it is discussed in its own
section below.

# Code Samples

The generalization of the Markdown image syntax into resources is one of
Markua's most important additions to Markdown. For example, the image syntax
is the inspiration for the syntax for external code samples:

![](hello.rb "Hello World in Ruby")

Just like with images, you can also use an attribute list for the title:

{title: "Hello World in Ruby"}
![](hello.rb)

Note that you can also specify the figure title as the alt text, as long as
the document setting of `alt-title` is not set to `none`:

![Hello World in Ruby](hello.rb)

You can also have inline code samples, which can only have a title using an
attribute list:

{title: "Hello World in Ruby"}
```ruby
puts "hello"
```

You can also include single lines of code like `puts "hello"` in paragraphs
using backticks.

# Other Stuff

Note that you can easily add math `d = v_i t + \frac{1}{2} a t^2`$ as well,
either inline in a paragraph or as a figure, using LaTeX math as the format.

> Blockquotes are really easy too.
> --Peter Armstrong, *Markua Spec*

Markua has lots of features not discussed in this example. Read the manual or
the rest of the spec!
~~~

</div>



<div class="extension">

## For Leanpub authors (M)

There are three ways to write in plain text on Leanpub. These are their
user manuals:

1. [Leanpub Flavoured Markdown (LFM)](https://leanpub.com/lfm/read) - LFM was
   launched in 2011, and evolved until 2013. It is now obsolete, and is on
   long-term support.
2. [Markua 0.10](https://leanpub.com/markua/read) - Markua 0.10 was launched in
   2014, and evolved until 2021. It is currently the only way to write courses
   on Leanpub.
3. [Markua 0.30 Beta](https://leanpub.com/markua030/read) - The Markua 0.30
   beta was launched on Leanpub in 2021. Currently, Markua 0.30 is the default
   for new books on Leanpub. The user manual for Markua 0.30 is not yet stable,
   so for now you're much better off reading this spec!

Not all features in the Markua spec are supported in the Markua 0.30 beta.
However, the Markua 0.30 beta is good enough that it is the default choice for
new Leanpub books!

You can still choose to use Leanpub's Markua 0.10 support, of course. This is
also true if you want to create courses. Leanpub's Markua 0.30 implementation
does not support courses yet.

You can also choose to use LFM, but unless you're dealing with a legacy Leanpub
book, there's no good reason to do so. You're much better off using the
Markua 0.30 beta than LFM.

Markua 0.30 is compared to LFM and Markua 0.10 in the next sections below.
These sections are primarily here for the benefit of Leanpub authors, but also
to show the origin of how Markua 0.30 evolved.

</div>



<div class="extension">

## Markua 0.30 vs. Leanpub Flavoured Markdown (LFM) (M)

If you have used Leanpub Flavoured Markdown (LFM) in the past, you may be
curious about the main differences between Markua 0.30 and LFM.

First, to emphasize the most obvious point: **If you are writing a new book or
course on Leanpub, you should absolutely choose Markua over LFM.** For new
books, you should choose Markua 0.30. For new courses, you should choose Markua
0.10 for now, since we do not support Markua 0.30 for courses yet.

**There is no good reason to start any new project using LFM.** The only reason
we continue to support LFM on Leanpub is for updates to legacy books.

There are many differences between Markua 0.30 and LFM. This list highlights 10
of the more important ones...

1. In LFM, parts are created with `-# Part`. In Markua, parts are created with
   a `{class: part}` attribute list above an atx (`# Foo`) or Setext (using
   `===`) chapter heading.
2. In LFM, there is a special syntax for inserting code samples:
   `<<[Optional Title](code/some_code_file.rb)`. In Markua, however, code
   is just a resource, and the LFM syntax is not supported.
3. In LFM, to mark code as added or removed, the magic words were
   `leanpub-start-insert`, `leanpub-end-insert`, `leanpub-start-delete` and
   `leanpub-end-delete`. In Markua, the magic words are `markua-start-insert`,
   `markua-end-insert`, `markua-start-delete` and `markua-end-delete`.
4. In LFM, there is a special syntax for inserting math: `{$$}...{/$$}`. This
   looks nice to people who like LaTeX, and looks like nothing else in
   Markdown. In Markua, however, math is just another resource, and that
   LaTeX-inspired syntax for wrapping math resources is not supported.
5. In LFM, there are `G>` "generic boxes". In Markua, these are replaced with
   blurbs (`B>`).
6. LFM had the `C>` syntax to center text, but we didn't have the same effect on
   generic boxes, and blurbs did not exist. In Markua, a `C>` syntax is just
   syntactic sugar for a centered blurb, for greater consistency. Because of
   this, the blurb also gets the ability to be centered by adding a
   `{class: center}` attribute.
7. LFM had `{rtl}` and `{ltr}` directives. These are not supported in Markua,
   and neither is a `{dir}` attribute in general: any given language is either a
   left-to-right or a right-to-left language, so specifying the language with
   the `lang` document setting and the `{lang: ___}` directive is sufficient.
8. LFM used Sample.txt to define the sample content. Markua moves the definition
   of what constitutes sample content into a `{sample: true}` attribute on
   parts, chapters and sections. So, in Markua, inclusion in the sample is at
   the content level, not the file level. This helps avoid a number of bugs that
   could happen with including at the file level, if a file did not clearly
   break at a chapter boundary. (So, in Leanpub, the Sample.txt approach is not
   supported for books which use Markua.)
9. LFM used Book.txt to define the book content. In Markua, the way that the
   list of manuscript files is defined is considered out of scope of the Markua
   specification itself. (Leanpub still uses the Book.txt approach to specify
   which files are in a book, but other Markua Processors could use other
   approaches, or could just support parsing single files or input streams.)
10. Most importantly, a number of features (courses, index entries, smart
    crosslinks, etc.) exist in Markua and do not exist in LFM.

</div>



<div class="extension">

## Migrating a Leanpub Flavoured Markdown (LFM) Book to Markua 0.30 (M)

Migrating a Leanpub Flavoured Markdown (LFM) Book to Markua 0.30 is actually
really easy.

Here are the steps you do:

1. Move the images folder inside a resources folder.
2. Convert the part syntax, if you use parts.
3. Generate the book, look at the output, and make any other changes needed.

These are all described below.

### Move the images folder inside a resources folder

In LFM, images live here:

```
manuscript/images/some_image.png
```

To convert this to Markua 0.30, the simplest thing to do is just move the images
folder inside the resources folder:

```
manuscript/resources/images/some_image.png
```

This way, your image paths will be unchanged (as they will both start
with `images/`).

### Convert the part syntax, if you use parts

In LFM, parts are defined like this:

```
-# Some Part Title
```

To convert this to Markua 0.30, you do this:

```
{class: part}
# Some Part Title
```

### Generate the book, look at the output, and make any other changes needed

The above list of changes are the main ones, but there are
[other differences](#markua-0-30-vs-leanpub-flavoured-markdown-lfm-m-) between
LFM and Markua 0.30.

However, at this point, the best thing to do is to just generate the book using
Markua 0.30 instead of LFM, and see what is produced. Then, consult that list
and the [LFM manual](https://leanpub.com/lfm/read) to learn more about how to
make any other changes needed to finish the migration.

One thing that Markua 0.30 defines which is not done in LFM is the concept of
document settings, which are a set of global switches which customize the
output of your book. You will want to review the
[standard document settings](#the-standard-document-settings-m-) section below
to learn more.

</div>



<div class="extension">

## Markua 0.30 vs. Markua 0.10 (M)

If you are writing a new book on Leanpub, you should absolutely choose
Markua 0.30 over Markua 0.10.

If you are writing a new course on Leanpub, you're stuck with Markua 0.10
for a while longer: our Markua 0.30 support don't support courses yet.

Leanpub's Markua 0.10 support is fully documented in its own [user
manual](https://leanpub.com/markua/read). Everything documented in that
manual should work as described for Markua 0.10 books and courses, and our
Markua 0.10 version will continue to be supported for years to come.

Leanpub's Markua 0.10 support is based on a private fork of an open source
Ruby library. It is not based on CommonMark or Pandoc. So, there are a
number of quirks to our Markua 0.10 support. If you are using Markua 0.10
on Leanpub, consult the [user manual](https://leanpub.com/markua/read), NOT
this spec.

</div>



<div class="extension">

## Migrating a Markua 0.10 Book to Markua 0.30 (M)

When I defined how Markua 0.10 worked, I was a number of years younger than I
am now. I cared less about compatibility with Markdown defaults, and more about
trying to fix what I thought were mistakes.

This was youthful hubris, which Markua 0.30 corrects.

What Markua 0.30 does is functions like Markdown (minus inline HTML, and
then extended) by default, but also provides the ability to override
behavior using global [document settings](#document-settings-m-). These
document settings allow you to customize the behavior of an entire Markua
document via global switches. This way, the behavior of a Markua 0.30
document can be changed in sweeping ways via these global settings.

If you are updating a large Markua 0.10 document to Markua 0.30, you will
probably want to start by doing these three steps:

1. Rename all `caption` attributes to `title`
2. Preview the book
3. Update any document settings

These are all described below.

### Rename all `caption` attributes to `title`

First, you will want to do a find-replace in your Markua manuscript to update
all `caption:` attributes to be `title:`, since the `caption` attribute was
renamed to `title` in Markua 0.30 for consistency.

Now, if you're like most Leanpub authors who wrote in Markua 0.10, chances
are you did not use any `caption` attributes, but instead relied on the
`alt-title` behavior. The `alt-title` document setting is discussed
[in great detail here](#the-alt-title-document-setting-m-).

### Preview the book

Before making any more changes, you should preview your book to see what it
looks like unchanged.

### Update any document settings

After previewing the book, you will want to review the PDF output and decide on
the appropriate values of the document settings.

If you want maximum compatibility with Markua 0.10, you will want to override
the `soft-breaks` and `two-space-hack` settings as follows:

1. Set the `soft-breaks` setting to `break`
2. Set the `two-space-hack` setting to `false`

Note that both of these settings are against the default behavior of Markdown,
so unless you are already relying on that behaviour in Markua 0.10, you should
consider leaving them at their default values.

You can change these settings on Leanpub in the Settings > Generation Settings
section of the author app, or you can set the document settings at the top of
the first manuscript file like this:

```
{
    soft-breaks: break
    two-space-hack: false
}

# Chapter One

Lorem ipsum dolor...
```

Make sure you add a blank line below the document settings, and also make sure
you put the curly braces on lines by themselves!

Note that you only define this list of document settings **once**, at the start
of your **first file** in your Markua manuscript.

Besides the specifics of the `soft-breaks` and the `two-space-hack`
document settings, the big point is that regardless of whether you are
upgrading a large Markua 0.10 document to Markua 0.30 or starting a new
Markua 0.30 document, you can make sweeping changes to how a Markua 0.30
document functions with a little bit of metadata at the top.

I strongly recommend that you review all of
the [document settings](#document-settings-m-) to learn what choices are
available, and decide which make sense for you. There's no one true right
answer here; it's entirely based on personal preference and whether you are
starting a new book or upgrading an existing one.

</div>



<div class="extension">

## Origin and structure of this document (M)

This is the Markua Spec, written by
[Peter Armstrong](https://twitter.com/peterarmstrong).

The Markua Spec is based on the CommonMark Spec, whose author is
[John MacFarlane](https://johnmacfarlane.net/).

This document also incorporates the specification of two of the five GitHub
Flavored Markdown (GFM) extensions from the [GFM
Spec](https://github.github.com/gfm/).

Like the CommonMark Spec and the GFM Spec, the Markua Spec is licensed under
the Creative Commons license
[CC-BY-SA 4.0](https://creativecommons.org/licenses/by-sa/4.0/).

This document has two types of intended readers:

1. authors with advanced questions about Markua syntax
2. developers implementing Markua Processors

The Markua Spec is a strange hybrid of the CommonMark Spec, the GFM Spec and the
specification of the Markua extensions.

The starting point is the CommonMark Spec. With the exception of the removal of
the HTML Blocks and Raw HTML sections, the entire CommonMark Spec is preserved
unchanged.

Next, two of the five GFM extensions are added. These are discussed
[below](#github-flavored-markdown-gfm-extension-status-m-).

Finally, many new chapters or sections about Markua are added. These are all
labeled with "(Markua extension)".

So, for any given chapter or section of this document, it is either:

1. written by John MacFarlane about CommonMark
2. written by GitHub employees about GFM, and ending with "(GFM)"
3. written by Peter Armstrong about Markua, and ending with "(M)"

To be clear: every chapter or section which does not end with (M) or (GFM) is
from the original CommonMark Spec, and was written by John MacFarlane.

Together, these sections combine to specify Markua, since Markua is CommonMark,
minus HTML blocks and Raw HTML, plus two GFM extensions, plus many Markua
extensions.

While this hybrid approach of creating the Markua specification is somewhat odd,
it has a number of significant benefits:

1. The CommonMark Spec is very good, and is licensed under the Creative Commons
   license CC-BY-SA 4.0. Not starting with the CommonMark Spec would be an
   exercise in vanity.
2. The CommonMark Spec is the defacto standard specification of Markdown. Any
   specification not based on CommonMark will need to be compared to CommonMark,
   so the most helpful thing to do is to just start with CommonMark.
3. Writing this specification as a series of extensions to CommonMark will
   enable people who are implementing it to just start with a compliant
   CommonMark implementation and modify it appropriately.
4. Defining all Markua changes as extensions to CommonMark or deletions from
   CommonMark makes it abundantly clear whether something that Markua does
   originates in CommonMark, or whether it was added by GFM or by Markua. This
   is potentially helpful for authors, since it will show them what will work
   in other Markdown dialects, and what won't.
5. It means that updating this document to keep it current with the CommonMark
   and GFM specs will be very easy.

In terms of the GFM extensions, the following two GFM extensions are included
in the Markua Spec:

1. [Tables (GFM)](#tables-gfm-)
2. [Strikethrough (GFM)](#strikethrough-gfm-)

Their content is unchanged, but their titles are renamed to say "(GFM
extension)" instead of "(extension)", to more clearly specify their provenance.

The other three GFM extensions are omitted:

1. [Task list items](https://github.github.com/gfm/#task-list-items-extension-)
   is omitted since Markua is for books and courses, not task lists.
2. [Disallowed Raw HTML](https://github.github.com/gfm/#disallowed-raw-html-extension-)
   is omitted since Markua goes farther and **completely removes all raw HTML**,
   both HTML added as HTML blocks or as raw HTML inlines.
3. [Autolinks](https://github.github.com/gfm/#autolinks-extension-) is omitted,
   because the extra work to create a link is literally typing two characters
   (`<` and `>`), so the added complexity both in parsing and in documentation
   is not worth it. (It does make sense for, say, a discussion forum. It does
   not make sense for books and courses.)

</div>



<div class="extension">

## About the HTML output (M)

The HTML mapping defined in this specification is NOT a complete specification
of the HTML documents produced by Markua Processors.

Also, the HTML mapping defined in the specification is NOT considered to be
canonical.

For example, consider the following Markua:

```
![a red apple](mac.jpg "The Original Mac")
```

A Markua Processor can produce HTML that looks like this:

```
<p><img src="mac.jpg" alt="a red apple" title="The Original Mac" /></p>
```

(That is what was produced at <https://spec.commonmark.org/dingus/> on
2020-05-27, for example, and it is valid Markua as well as valid CommonMark.)

A Markua Processor can also, however, produce HTML that looks like this:

```
<p>
  <figure>
    <img src="mac.jpg"
         alt="a red apple">
    <figcaption>The Original Mac</figcaption>
  </figure>
</p>
```

Both approaches are completely legitimate, and have pros and cons.

So, why show any HTML at all? There are two reasons:

1. The CommonMark Spec shows HTML, and the Markua Spec tries to only be additive
   to the CommonMark Spec.
2. Showing HTML helps Markua Processors to test their parsers.

Finally, besides not being canonical, the HTML is not complete. It is only a
specification of the parts of the HTML mapping where there is less need to have
flexibility on the part of the Markua Processor.

The HTML mapping should be thought of specifying HTML fragments, not HTML
documents. In all of the examples, a Markua Processor may add more HTML before
and after the relevant content, as well as changing the relevant content itself.

If a Markua Processor wishes to test that it produces the correct HTML, it
should test that the HTML produced contains the specified output, not that it
is identical to the specified output.

Every example uses the default Markua document settings, unless otherwise
specified. Any custom settings are specified in a document settings list in the
top of the example.

</div>



<div class="extension">

## Why the name "Markua"? (M)

When I set out to specify Markua, I realized I needed a name. I wanted a name
that conveyed the love that I have for Markdown while not implying endorsement
by John Gruber in any way. I also did not want a name which referenced Leanpub:
Markua is a standalone specification with its own identity, which anyone
(including Leanpub competitors) can freely implement. Finally, I was on vacation
in Hawaii when I named Markua, and I wanted something that sounded happy,
friendly and almost Hawaiian. (Yes, I'm aware that there is no r in Hawaiian.)
I also wanted a name that had its .com domain name available, and that was short
and spellable, for branding purposes. The Markua name had all these properties.

So, I named it Markua, registered the domain name, and filed a trademark.
Markua and the Markua logo is a registered trademark of Ruboss Technology
Corporation, the company which created, owns and operates Leanpub.

</div>



## What is Markdown?

Markdown is a plain text format for writing structured documents,
based on conventions for indicating formatting in email
and usenet posts.  It was developed by John Gruber (with
help from Aaron Swartz) and released in 2004 in the form of a
[syntax description](https://daringfireball.net/projects/markdown/syntax)
and a Perl script (`Markdown.pl`) for converting Markdown to
HTML.  In the next decade, dozens of implementations were
developed in many languages.  Some extended the original
Markdown syntax with conventions for footnotes, tables, and
other document elements.  Some allowed Markdown documents to be
rendered in formats other than HTML.  Websites like Reddit,
StackOverflow, and GitHub had millions of people using Markdown.
And Markdown started to be used beyond the web, to author books,
articles, slide shows, letters, and lecture notes.

What distinguishes Markdown from many other lightweight markup
syntaxes, which are often easier to write, is its readability.
As Gruber writes:

> The overriding design goal for Markdown's formatting syntax is
> to make it as readable as possible. The idea is that a
> Markdown-formatted document should be publishable as-is, as
> plain text, without looking like it's been marked up with tags
> or formatting instructions.
> (<https://daringfireball.net/projects/markdown/>)

The point can be illustrated by comparing a sample of
[AsciiDoc](https://asciidoc.org/) with
an equivalent sample of Markdown.  Here is a sample of
AsciiDoc from the AsciiDoc manual:

```
1. List item one.
+
List item one continued with a second paragraph followed by an
Indented block.
+
.................
$ ls *.sh
$ mv *.sh ~/tmp
.................
+
List item continued with a third paragraph.

2. List item two continued with an open block.
+
--
This paragraph is part of the preceding list item.

a. This list is nested and does not require explicit item
continuation.
+
This paragraph is part of the preceding list item.

b. List item b.

This paragraph belongs to item two of the outer list.
--
```

And here is the equivalent in Markdown:
```
1.  List item one.

    List item one continued with a second paragraph followed by an
    Indented block.

        $ ls *.sh
        $ mv *.sh ~/tmp

    List item continued with a third paragraph.

2.  List item two continued with an open block.

    This paragraph is part of the preceding list item.

    1. This list is nested and does not require explicit item continuation.

       This paragraph is part of the preceding list item.

    2. List item b.

    This paragraph belongs to item two of the outer list.
```

The AsciiDoc version is, arguably, easier to write. You don't need
to worry about indentation.  But the Markdown version is much easier
to read.  The nesting of list items is apparent to the eye in the
source, not just in the processed document.

## Why is a spec needed?

John Gruber's [canonical description of Markdown's
syntax](https://daringfireball.net/projects/markdown/syntax)
does not specify the syntax unambiguously.  Here are some examples of
questions it does not answer:

1.  How much indentation is needed for a sublist?  The spec says that
    continuation paragraphs need to be indented four spaces, but is
    not fully explicit about sublists.  It is natural to think that
    they, too, must be indented four spaces, but `Markdown.pl` does
    not require that.  This is hardly a "corner case," and divergences
    between implementations on this issue often lead to surprises for
    users in real documents. (See [this comment by John
    Gruber](https://web.archive.org/web/20170611172104/http://article.gmane.org/gmane.text.markdown.general/1997).)

2.  Is a blank line needed before a block quote or heading?
    Most implementations do not require the blank line.  However,
    this can lead to unexpected results in hard-wrapped text, and
    also to ambiguities in parsing (note that some implementations
    put the heading inside the blockquote, while others do not).
    (John Gruber has also spoken [in favor of requiring the blank
    lines](https://web.archive.org/web/20170611172104/http://article.gmane.org/gmane.text.markdown.general/2146).)

3.  Is a blank line needed before an indented code block?
    (`Markdown.pl` requires it, but this is not mentioned in the
    documentation, and some implementations do not require it.)

    ``` markdown
    paragraph
        code?
    ```

4.  What is the exact rule for determining when list items get
    wrapped in `<p>` tags?  Can a list be partially "loose" and partially
    "tight"?  What should we do with a list like this?

    ``` markdown
    1. one

    2. two
    3. three
    ```

    Or this?

    ``` markdown
    1.  one
        - a

        - b
    2.  two
    ```

    (There are some relevant comments by John Gruber
    [here](https://web.archive.org/web/20170611172104/http://article.gmane.org/gmane.text.markdown.general/2554).)

5.  Can list markers be indented?  Can ordered list markers be right-aligned?

    ``` markdown
     8. item 1
     9. item 2
    10. item 2a
    ```

6.  Is this one list with a thematic break in its second item,
    or two lists separated by a thematic break?

    ``` markdown
    * a
    * * * * *
    * b
    ```

7.  When list markers change from numbers to bullets, do we have
    two lists or one?  (The Markdown syntax description suggests two,
    but the perl scripts and many other implementations produce one.)

    ``` markdown
    1. fee
    2. fie
    -  foe
    -  fum
    ```

8.  What are the precedence rules for the markers of inline structure?
    For example, is the following a valid link, or does the code span
    take precedence ?

    ``` markdown
    [a backtick (`)](/url) and [another backtick (`)](/url).
    ```

9.  What are the precedence rules for markers of emphasis and strong
    emphasis?  For example, how should the following be parsed?

    ``` markdown
    *foo *bar* baz*
    ```

10. What are the precedence rules between block-level and inline-level
    structure?  For example, how should the following be parsed?

    ``` markdown
    - `a long code span can contain a hyphen like this
      - and it can screw things up`
    ```

11. Can list items include section headings?  (`Markdown.pl` does not
    allow this, but does allow blockquotes to include headings.)

    ``` markdown
    - # Heading
    ```

12. Can list items be empty?

    ``` markdown
    * a
    *
    * b
    ```

13. Can link references be defined inside block quotes or list items?

    ``` markdown
    > Blockquote [foo].
    >
    > [foo]: /url
    ```

14. If there are multiple definitions for the same reference, which takes
    precedence?

    ``` markdown
    [foo]: /url1
    [foo]: /url2

    [foo][]
    ```

In the absence of a spec, early implementers consulted `Markdown.pl`
to resolve these ambiguities.  But `Markdown.pl` was quite buggy, and
gave manifestly bad results in many cases, so it was not a
satisfactory replacement for a spec.

Because there is no unambiguous spec, implementations have diverged
considerably.  As a result, users are often surprised to find that
a document that renders one way on one system (say, a GitHub wiki)
renders differently on another (say, converting to docbook using
pandoc).  To make matters worse, because nothing in Markdown counts
as a "syntax error," the divergence often isn't discovered right away.

## About this document

This document attempts to specify Markdown syntax unambiguously.
It contains many examples with side-by-side Markdown and
HTML.  These are intended to double as conformance tests.  An
accompanying script `spec_tests.py` can be used to run the tests
against any Markdown program:

    python test/spec_tests.py --spec spec.txt --program PROGRAM

Since this document describes how Markdown is to be parsed into
an abstract syntax tree, it would have made sense to use an abstract
representation of the syntax tree instead of HTML.  But HTML is capable
of representing the structural distinctions we need to make, and the
choice of HTML for the tests makes it possible to run the tests against
an implementation without writing an abstract syntax tree renderer.

Note that not every feature of the HTML samples is mandated by
the spec.  For example, the spec says what counts as a link
destination, but it doesn't mandate that non-ASCII characters in
the URL be percent-encoded.  To use the automatic tests,
implementers will need to provide a renderer that conforms to
the expectations of the spec examples (percent-encoding
non-ASCII characters in URLs).  But a conforming implementation
can use a different renderer and may choose not to
percent-encode non-ASCII characters in URLs.

This document is generated from a text file, `spec.txt`, written
in Markdown with a small extension for the side-by-side tests.
The script `tools/makespec.py` can be used to convert `spec.txt` into
HTML or CommonMark (which can then be converted into other formats).

In the examples, the `→` character is used to represent tabs.

# Preliminaries

## Characters and lines

Any sequence of [characters] is a valid CommonMark
document.

A [character](@) is a Unicode code point.  Although some
code points (for example, combining accents) do not correspond to
characters in an intuitive sense, all code points count as characters
for purposes of this spec.

This spec does not specify an encoding; it thinks of lines as composed
of [characters] rather than bytes.  A conforming parser may be limited
to a certain encoding.

A [line](@) is a sequence of zero or more [characters]
other than line feed (`U+000A`) or carriage return (`U+000D`),
followed by a [line ending] or by the end of file.

A [line ending](@) is a line feed (`U+000A`), a carriage return
(`U+000D`) not followed by a line feed, or a carriage return and a
following line feed.

A line containing no characters, or a line containing only spaces
(`U+0020`) or tabs (`U+0009`), is called a [blank line](@).

The following definitions of character classes will be used in this spec:

A [Unicode whitespace character](@) is a character in the Unicode `Zs` general
category, or a tab (`U+0009`), line feed (`U+000A`), form feed (`U+000C`), or
carriage return (`U+000D`).

[Unicode whitespace](@) is a sequence of one or more
[Unicode whitespace characters].

A [tab](@) is `U+0009`.

A [space](@) is `U+0020`.

An [ASCII control character](@) is a character between `U+0000–1F` (both
including) or `U+007F`.

An [ASCII punctuation character](@)
is `!`, `"`, `#`, `$`, `%`, `&`, `'`, `(`, `)`,
`*`, `+`, `,`, `-`, `.`, `/` (U+0021–2F), 
`:`, `;`, `<`, `=`, `>`, `?`, `@` (U+003A–0040),
`[`, `\`, `]`, `^`, `_`, `` ` `` (U+005B–0060), 
`{`, `|`, `}`, or `~` (U+007B–007E).

A [Unicode punctuation character](@) is a character in the Unicode `P`
(puncuation) or `S` (symbol) general categories.

## Tabs

Tabs in lines are not expanded to [spaces].  However,
in contexts where spaces help to define block structure,
tabs behave as if they were replaced by spaces with a tab stop
of 4 characters.

Thus, for example, a tab can be used instead of four spaces
in an indented code block.  (Note, however, that internal
tabs are passed through as literal tabs, not expanded to
spaces.)

```````````````````````````````` example
→foo→baz→→bim
.
<pre><code>foo→baz→→bim
</code></pre>
````````````````````````````````

```````````````````````````````` example
  →foo→baz→→bim
.
<pre><code>foo→baz→→bim
</code></pre>
````````````````````````````````

```````````````````````````````` example
    a→a
    ὐ→a
.
<pre><code>a→a
ὐ→a
</code></pre>
````````````````````````````````

In the following example, a continuation paragraph of a list
item is indented with a tab; this has exactly the same effect
as indentation with four spaces would:

```````````````````````````````` example
  - foo

→bar
.
<ul>
<li>
<p>foo</p>
<p>bar</p>
</li>
</ul>
````````````````````````````````

```````````````````````````````` example
- foo

→→bar
.
<ul>
<li>
<p>foo</p>
<pre><code>  bar
</code></pre>
</li>
</ul>
````````````````````````````````

Normally the `>` that begins a block quote may be followed
optionally by a space, which is not considered part of the
content.  In the following case `>` is followed by a tab,
which is treated as if it were expanded into three spaces.
Since one of these spaces is considered part of the
delimiter, `foo` is considered to be indented six spaces
inside the block quote context, so we get an indented
code block starting with two spaces.

```````````````````````````````` example
>→→foo
.
<blockquote>
<pre><code>  foo
</code></pre>
</blockquote>
````````````````````````````````

```````````````````````````````` example
-→→foo
.
<ul>
<li>
<pre><code>  foo
</code></pre>
</li>
</ul>
````````````````````````````````


```````````````````````````````` example
    foo
→bar
.
<pre><code>foo
bar
</code></pre>
````````````````````````````````

```````````````````````````````` example
 - foo
   - bar
→ - baz
.
<ul>
<li>foo
<ul>
<li>bar
<ul>
<li>baz</li>
</ul>
</li>
</ul>
</li>
</ul>
````````````````````````````````

```````````````````````````````` example
#→Foo
.
<h1>Foo</h1>
````````````````````````````````

```````````````````````````````` example
*→*→*→
.
<hr />
````````````````````````````````


## Insecure characters

For security reasons, the Unicode character `U+0000` must be replaced
with the REPLACEMENT CHARACTER (`U+FFFD`).


## Backslash escapes

Any ASCII punctuation character may be backslash-escaped:

```````````````````````````````` example
\!\"\#\$\%\&\'\(\)\*\+\,\-\.\/\:\;\<\=\>\?\@\[\\\]\^\_\`\{\|\}\~
.
<p>!&quot;#$%&amp;'()*+,-./:;&lt;=&gt;?@[\]^_`{|}~</p>
````````````````````````````````


Backslashes before other characters are treated as literal
backslashes:

```````````````````````````````` example
\→\A\a\ \3\φ\«
.
<p>\→\A\a\ \3\φ\«</p>
````````````````````````````````


Escaped characters are treated as regular characters and do
not have their usual Markdown meanings:

```````````````````````````````` example
\*not emphasized*
\<br/> not a tag
\[not a link](/foo)
\`not code`
1\. not a list
\* not a list
\# not a heading
\[foo]: /url "not a reference"
\&ouml; not a character entity
.
<p>*not emphasized*
&lt;br/&gt; not a tag
[not a link](/foo)
`not code`
1. not a list
* not a list
# not a heading
[foo]: /url &quot;not a reference&quot;
&amp;ouml; not a character entity</p>
````````````````````````````````


If a backslash is itself escaped, the following character is not:

```````````````````````````````` example
\\*emphasis*
.
<p>\<em>emphasis</em></p>
````````````````````````````````


A backslash at the end of the line is a [hard line break]:

```````````````````````````````` example
foo\
bar
.
<p>foo<br />
bar</p>
````````````````````````````````


Backslash escapes do not work in code blocks, code spans, autolinks, or
raw HTML:

```````````````````````````````` example
`` \[\` ``
.
<p><code>\[\`</code></p>
````````````````````````````````


```````````````````````````````` example
    \[\]
.
<pre><code>\[\]
</code></pre>
````````````````````````````````


```````````````````````````````` example
~~~
\[\]
~~~
.
<pre><code>\[\]
</code></pre>
````````````````````````````````


```````````````````````````````` example
<https://example.com?find=\*>
.
<p><a href="https://example.com?find=%5C*">https://example.com?find=\*</a></p>
````````````````````````````````


```````````````````````````````` example
<a href="/bar\/)">
.
<a href="/bar\/)">
````````````````````````````````


But they work in all other contexts, including URLs and link titles,
link references, and [info strings] in [fenced code blocks]:

```````````````````````````````` example
[foo](/bar\* "ti\*tle")
.
<p><a href="/bar*" title="ti*tle">foo</a></p>
````````````````````````````````


```````````````````````````````` example
[foo]

[foo]: /bar\* "ti\*tle"
.
<p><a href="/bar*" title="ti*tle">foo</a></p>
````````````````````````````````


```````````````````````````````` example
``` foo\+bar
foo
```
.
<pre><code class="language-foo+bar">foo
</code></pre>
````````````````````````````````


## Entity and numeric character references

Valid HTML entity references and numeric character references
can be used in place of the corresponding Unicode character,
with the following exceptions:

- Entity and character references are not recognized in code
  blocks and code spans.

- Entity and character references cannot stand in place of
  special characters that define structural elements in
  CommonMark.  For example, although `&#42;` can be used
  in place of a literal `*` character, `&#42;` cannot replace
  `*` in emphasis delimiters, bullet list markers, or thematic
  breaks.

Conforming CommonMark parsers need not store information about
whether a particular character was represented in the source
using a Unicode character or an entity reference.

[Entity references](@) consist of `&` + any of the valid
HTML5 entity names + `;`. The
document <https://html.spec.whatwg.org/entities.json>
is used as an authoritative source for the valid entity
references and their corresponding code points.

```````````````````````````````` example
&nbsp; &amp; &copy; &AElig; &Dcaron;
&frac34; &HilbertSpace; &DifferentialD;
&ClockwiseContourIntegral; &ngE;
.
<p>  &amp; © Æ Ď
¾ ℋ ⅆ
∲ ≧̸</p>
````````````````````````````````


[Decimal numeric character
references](@)
consist of `&#` + a string of 1--7 arabic digits + `;`. A
numeric character reference is parsed as the corresponding
Unicode character. Invalid Unicode code points will be replaced by
the REPLACEMENT CHARACTER (`U+FFFD`).  For security reasons,
the code point `U+0000` will also be replaced by `U+FFFD`.

```````````````````````````````` example
&#35; &#1234; &#992; &#0;
.
<p># Ӓ Ϡ �</p>
````````````````````````````````


[Hexadecimal numeric character
references](@) consist of `&#` +
either `X` or `x` + a string of 1-6 hexadecimal digits + `;`.
They too are parsed as the corresponding Unicode character (this
time specified with a hexadecimal numeral instead of decimal).

```````````````````````````````` example
&#X22; &#XD06; &#xcab;
.
<p>&quot; ആ ಫ</p>
````````````````````````````````


Here are some nonentities:

```````````````````````````````` example
&nbsp &x; &#; &#x;
&#87654321;
&#abcdef0;
&ThisIsNotDefined; &hi?;
.
<p>&amp;nbsp &amp;x; &amp;#; &amp;#x;
&amp;#87654321;
&amp;#abcdef0;
&amp;ThisIsNotDefined; &amp;hi?;</p>
````````````````````````````````


Although HTML5 does accept some entity references
without a trailing semicolon (such as `&copy`), these are not
recognized here, because it makes the grammar too ambiguous:

```````````````````````````````` example
&copy
.
<p>&amp;copy</p>
````````````````````````````````


Strings that are not on the list of HTML5 named entities are not
recognized as entity references either:

```````````````````````````````` example
&MadeUpEntity;
.
<p>&amp;MadeUpEntity;</p>
````````````````````````````````


Entity and numeric character references are recognized in any
context besides code spans or code blocks, including
URLs, [link titles], and [fenced code block][] [info strings]:

```````````````````````````````` example
<a href="&ouml;&ouml;.html">
.
<a href="&ouml;&ouml;.html">
````````````````````````````````


```````````````````````````````` example
[foo](/f&ouml;&ouml; "f&ouml;&ouml;")
.
<p><a href="/f%C3%B6%C3%B6" title="föö">foo</a></p>
````````````````````````````````


```````````````````````````````` example
[foo]

[foo]: /f&ouml;&ouml; "f&ouml;&ouml;"
.
<p><a href="/f%C3%B6%C3%B6" title="föö">foo</a></p>
````````````````````````````````


```````````````````````````````` example
``` f&ouml;&ouml;
foo
```
.
<pre><code class="language-föö">foo
</code></pre>
````````````````````````````````


Entity and numeric character references are treated as literal
text in code spans and code blocks:

```````````````````````````````` example
`f&ouml;&ouml;`
.
<p><code>f&amp;ouml;&amp;ouml;</code></p>
````````````````````````````````


```````````````````````````````` example
    f&ouml;f&ouml;
.
<pre><code>f&amp;ouml;f&amp;ouml;
</code></pre>
````````````````````````````````


Entity and numeric character references cannot be used
in place of symbols indicating structure in CommonMark
documents.

```````````````````````````````` example
&#42;foo&#42;
*foo*
.
<p>*foo*
<em>foo</em></p>
````````````````````````````````

```````````````````````````````` example
&#42; foo

* foo
.
<p>* foo</p>
<ul>
<li>foo</li>
</ul>
````````````````````````````````

```````````````````````````````` example
foo&#10;&#10;bar
.
<p>foo

bar</p>
````````````````````````````````

```````````````````````````````` example
&#9;foo
.
<p>→foo</p>
````````````````````````````````


```````````````````````````````` example
[a](url &quot;tit&quot;)
.
<p>[a](url &quot;tit&quot;)</p>
````````````````````````````````


<div class="extension">

## Emoji (M)

In 2015, emoji fully arrived. The 2015 Oxford Dictionaries Word of the Year
was, in fact, the Face with Tears of Joy emoji. You may think of it as a
smiling face with tears of joy, but you can also can think of it as
`&#x1F602;`, which is the HTML entity reference to its Unicode character (in
hexadecimal).

However, Unicode characters aren't the only way to do emoji. Another popular
syntax for emoji is `:emoji_name:` -- that is a colon (`:`), followed by the
underscore-separated name of the emoji, followed by a colon.

Markua supports both the HTML character reference approach for emoji as well as
the `:emoji_name:` syntax.

Which emoji are supported is up to the Markua Processor.

One recommended list of emoji names is at
<https://www.webfx.com/tools/emoji-cheat-sheet/>.

The preferred thing to do when a Markua Processor recognizes an emoji is to
replace it with the Unicode code point HTML entity reference, assuming that
those are also handled correctly:

```````````````````````````````` example
This makes me &#x1F602;.

This makes me :joy:.
```
.
<p>This makes me &#x1F602;.</p>
<p>This makes me &#x1F602;.</p>
````````````````````````````````

However, it is also acceptable for a Markua Processor to just output the "tofu"
characters for the HTML entity references while still supporting the
`:emoji_name:` style emoji. Presumably with this approach, either a set of
images or an emoji font will be used for the supported emoji.

Finally, if Font Awesome icons are going to be supported by a Markua Processor,
using the `:emoji_name:` syntax, they must have their `:fa-` prefix which they
always come with. So, the [Leanpub logo in Font
Awesome](https://fontawesome.com/icons/leanpub?style=brands) would be output as
`:fa-leanpub:`.

</div>



<div class="extension">

# Document semantics (M)

As discussed in the introduction, Markua is an extension to Markdown. The goal
is for Markua to feel as though Markdown just magically grew a knowledge of
how to produce books and courses.

Recall that [Markua's five main additions to
Markdown](#markua-s-five-key-additions-to-markdown-m-) include the mapping of
Markdown headings to book structures, the unified attribute list syntax and a
comprehensive definition of document settings. These three key additions are
discussed in this section. (The other two key additions, resources and online
courses, are both big enough to need their own sections.

Markua adds a number of extensions to support metadata in a consistent way.
These include attributes (on everything), settings (at the beginning of a
document) and directives (at certain points throughout the document). All
metadata is enclosed in curly braces `{` ... `}`.

Metadata is not actually output in the Markua document itself. Instead, it
changes the behavior of the Markua Processor, sometimes dramatically.

First, however, it is important to start with how Markua headers and metadata
are used to define document structure.

## How headers define document structure (M)

Markua is a way of writing books and courses. Books have things like chapters,
sections and subsections. Courses typically call their chapters "modules", but
it's the same idea. Sometimes books or courses organize their chapters or
modules in parts.

Markdown defines two types of heading styles.

Markua's first innovation is to map both of these heading styles to document
structure concepts.

The preferred style, called atx headers, has the following meaning in Markua:

```
{class: part}
# Part

This is a paragraph.

# Chapter

This is a paragraph.

## Section

This is a paragraph.

### Sub-section

This is a paragraph.

#### Sub-sub-section

This is a paragraph.

##### Sub-sub-sub-section

This is a paragraph.

###### Sub-sub-sub-sub-section

This is a paragraph.
```

The other style of headers, called Setext headers, defines two levels of
heading only. Here's how those are used in Markua:

```
{class: part}
Part
====

This is a paragraph.

Chapter
=======

This is a paragraph.

Section
-------

This is a paragraph.
```

Setext headers look nice, but only if you're only using chapters and sections.
If you want to add sub-sections (or lower), you'll be using atx headers for at
least some of your headers. My advice is to just use atx headers all the time.
(The `{class: part}` attribute list on a chapter header to make a part header
does actually work with Setext headers, but it's really ugly.)

Note that while it is confusing and ugly to mix and match using atx and Setext
headers for chapters and sections in the same document, you can do it. However,
please don't.

Now, regardless of whether you are using the atx or Setext header style of
header, the key point is that Markua is mapping the headers to document
structure concepts. The header defines the name of the part, chapter, section,
etc., and the content underneath it becomes the content of that given thing
(until the next header is reached).

In the HTML output, the Part and Chapter heading (for atx or Setext headers)
both produce an `h1`, the `Section` heading produces an `h2`, and the
Sub-Section heading produces an `h3`. Lower levels go down to `h6`.

To be clear, these are the Markdown "atx" header mappings:

`#`
: Used for a Chapter heading when there is no `{class: part}` attribute list,
  and used for a Part heading when there is. This produces an `h1` in HTML.

`##`
: Used for a Section heading. This produces an `h2` in HTML.

`###`
: Used for a Sub-Section heading. This produces an `h3` in HTML.

`####`
: Used for a Sub-Sub-Section heading. This produces an `h4` in HTML.

`#####`
: Used for a Sub-Sub-Sub-Section heading. This produces an `h5` in HTML.

`######`
: Used for a Sub-Sub-Sub-Sub-Section heading. This produces an `h6` in HTML.

The reason that a Part heading and a Chapter heading both use the `#` symbol,
and are only differentiated by the presence of a `{class: part}` attribute list,
is that not all books have parts, so it would be awkward to start chapters with
the second-level heading.

```````````````````````````````` example
{class: part}
# Part

This is a paragraph.

# Chapter

This is a paragraph.

## Section

This is a paragraph.

### Sub-section

This is a paragraph.

#### Sub-sub-section

This is a paragraph.

##### Sub-sub-sub-section

This is a paragraph.

###### Sub-sub-sub-sub-section

This is a paragraph.
.
<h1 class="part">Part</h1>
<p>This is a paragraph.</p>
<h1>Chapter</h1>
<p>This is a paragraph.</p>
<h2>Section</h2>
<p>This is a paragraph.</p>
<h3>Sub-section</h3>
<p>This is a paragraph.</p>
<h4>Sub-sub-section</h4>
<p>This is a paragraph.</p>
<h5>Sub-sub-sub-section</h5>
<p>This is a paragraph.</p>
<h6>Sub-sub-sub-sub-section</h6>
<p>This is a paragraph.</p>
````````````````````````````````

These are the Setext header mappings:

```````````````````````````````` example
{class: part}
Part
====

This is a paragraph.

Chapter
-------

This is a paragraph.
.
<h1 class="part">Part</h1>
<p>This is a paragraph.</p>
<h1>Chapter</h1>
<p>This is a paragraph.</p>
````````````````````````````````

In the HTML output, the only difference between a part and chapter heading on
the heading itself is that there is a `class="part"` attribute shown. However,
obviously, the part and chapter headings are treated differently elsewhere, such
as in the table of contents of the book or the navigation structure of the
course.

The `{class: part}` is an example of an attribute list, which is another of
Markua's main additions to Markdown. There can be no blank line between the
attribute list and the header it is attached to. Attribute lists are discussed
in the Metadata section later.

Note that adding a `{class: part}` attribute list to any other heading level
than an `h1` is ignored by Markua.

Here's an example of this being incorrectly added to an atx section header,
which will have no effect:

```````````````````````````````` example
{class: part}
## Section Heading

Foo
.
<h2>Section Heading</h2>
<p>Foo</p>
````````````````````````````````

Here's an example of this being incorrectly added to a Setext section header,
which will also have no effect:

```````````````````````````````` example
{class: part}
Section Heading
---------------

Foo
.
<h2>Section Heading</h2>
<p>Foo</p>
````````````````````````````````



## Attribute lists (M)

Attribute lists are used to define everything from specify the language of code
blocks, add ids for crosslinking and even support extensions to Markua.

### Attribute list format

An attribute list is one or more key-value, comma-separated pairs:

`{one: v1, two: "v2", three: 'v3!', four: true, five: 0, six: 3.14, seven: a b}`

Note that you can skip the space between the colon and the value: the following
`{format: ruby}` and `{format:ruby}` both work. However, for consistency I
recommend always adding a space.

Note that attribute values can be in no quotes, in double quotes (`"`) or single
quotes (`'`). Whenever an attribute contains spaces, using either double or
single quotes is preferred to no quotes, but you can get away with using no
quotes as long as the attribute value does not contain a comma.

The choice of double or single quotes is mostly personal taste. However, inside
double quotes, a double quote must be backslash-escaped (`\"`); inside single
quotes, a single quote must be backslash-escaped (`\'`). So, if your attribute
value has a lot of double quotes, then it's more convenient to wrap it in
single quotes, and vice-versa.

Regardless of whether quotes are used, leading and trailing spaces are removed
from all attribute values, but internal spaces within the attribute values are
preserved.

An attribute list can be inserted into a Markua document in one of three ways:

1. Immediately above a block element (e.g. heading, figure, aside, blurb, quiz,
   etc.), with **one newline** (not a blank line) separating it from the block
   element.
2. Immediately after a span element (e.g. a word, italicized phrase, etc.) in
   normal paragraphs and in similarly-simple contexts, with **no spaces**
   separating it from the span element.
3. On a line by itself, with one blank line above and below it. In this format,
   the attribute list contains [directives](#directives).

One common use of attributes is to add `id` attributes to headings:

```````````````````````````````` example
{id: foo}
# Chapter Foo

foo
.
<h1 id="foo" class="chapter">Chapter Foo</h1>
<p>foo</p>
````````````````````````````````

You can also use an `{#id}` syntactic sugar:

```````````````````````````````` example
{#bar}
# Chapter Bar

bar
.
<h1 id="bar" class="chapter">Chapter Bar</h1>
<p>bar</p>
````````````````````````````````

One common use of attributes is to add `id` attributes to span elements:

```````````````````````````````` example
Here [is lorem]{id: lorem}.

This is ipsum{#ipsum}.
.
<p>Here <span id="lorem">is lorem</span>.</p>

<p>This is <span id="ipsum">ipsum</span>.</p>
````````````````````````````````

If there is an error in the syntax of an attribute list, or if the Markua
Processor does not support an attribute list in a given context, it should just
ignore the attribute list and add an appropriate error.

Any line outside of a code resource which starts with an opening curly brace `{`
and ends with a closing curly brace `}` is assumed to be an attribute list, and
will not be output by a Markua Processor. If you want to start a line with a
literal opening curly brace `{` you need to preface it with a backslash (`\`).

You cannot add attribute lists inside headers:

```````````````````````````````` example
# Chapter Foo{id: foo}

foo
.
<h1 class="chapter">Chapter Foo</h1>
<p>foo</p>
````````````````````````````````

### Attribute Keys

The keys of attributes must consist exclusively of lowercase letters,
hyphens (`-`) and underscores (`_`). Uppercase letters are not permitted in
attribute keys: a Markua Processor must treat uppercase letters in attribute
keys as an error.

If a key is duplicated in an attribute list, the first key value is used and
subsequent ones are ignored. A Markua Processor should add a warning in its
list of warnings, which are *not* output in the output itself.

To be clear, the keys of attributes have nothing to do with HTML attributes,
even though HTML is one of the output formats of Markua. Markua does not have
any particular knowledge of HTML attributes; Markua understands Markua
attributes. Some Markua attributes, like `class`, happen to have the same name
as some attributes in HTML. This is just coincidence, and many others do not.

### Attribute Values

All attributes are text. Markua Processors should interpret text values of "true"
and "false" as representing true and false. Quotes, by which I mean double quotes
(`"`) not single quotes (`'`), are optional for attribute values, and are only
needed if the attribute value contains whitespace or special characters.

If a text attribute value contains a quote, it must be "escaped" with a
backslash: e.g. `{title: "\"Fresh\" Fish"}`

### id Attributes

As previously discussed, there is special syntactic sugar for ids: `{#foo}` is
equivalent to `{id: foo}`. However, ids are just attributes.

### title Attributes

Markua headings (part, chapter, section, sub-section, etc.) and figures can all
have `title` attributes specified in an attribute list. This is text which
overrides what is displayed for the heading or figure in the table of contents.
For a heading, it is analogous to the `title` attribute on a resource inserted
as a figure, which specifies the text to use for the figure in the appropriate
list of figures (e.g. List of Illustrations, Table of Tables, etc.). If a
heading does not have a title attribute, the text of the heading itself is
used--which is quite often exactly what is desired. Use of a `title` attribute
is always optional; it's only used when the default behavior of using the
heading text (or the `title` attribute for a resource) is not appropriate,
say if it's too long.

### class Attributes

One of the attributes which is supported in every attribute list is the `class`
attribute. This allows a Markua Processor to do smart things about formatting.

For example, if a chapter has a `{class: bibliography}` added to it, then
the Markua Processor can then use this information to change the indentation
of paragraphs within it to be hanging paragraphs.

### Conditional Inclusion Attributes on Headings

Markua headings (and *only* headings) may have various attributes which specify
which output formats their content (of the part, chapter, section, sub-section,
etc.) should be included in. If the given attribute is not present, the default
value of it is that specified by the nearest ancestor heading. If no such
attribute is present at a top-level heading, the default is given by the default
value for the attribute defined of Markua.

`cascade`
: `true` or `false`. The default is `true`. If `true`, the values of `sample`
and `full` cascade to the children of the given part, chapter, section,
sub-section, etc. If `false`, they do not. Typically you will want `cascade`
to be true in all cases except for parts. Since `cascade` is so coupled with
`full` and `sample`, this is discussed below.

`full`
: `true` or `false`. The default is `true`. If `true`, include this heading and
its content (including nested sections, subsections, etc.) in the full book or
course, including the PDF and EPUB versions and the web version that is being
generated. If `false`, omit it. Setting this to `false` is an easy way to
"comment out" a section of your book or course.

`sample`
: `true` or `false`. The default is `false`. If `true`, include the content
under this heading (including nested sections, subsections, etc.) in the sample
of the book or course that is being generated. If `false`, omit it.
Since the default is `false`, by default a sample book or course is empty.
Note that this attribute just governs the inclusion of the content, not the
heading itself. A Markua Processor may choose to omit all headings with
`sample: false` (either explicitly set or defaulted to false) or it may choose
to include every heading in the sample version of a book or course, in order
to produce a representative Table of Contents. In a case such as this where
`sample` is `false`, the Markua Processor may output special content inside the
chapter, section or subsection to indicate that the content itself is being
omitted from the sample. This attribute applies to both the book version
(PDF and EPUB) and the web version of the sample book or course.

Note that specifying either of these attributes in a nested section overrides any
value inherited from its ancestors, or from the default. This way, you can
include a chapter in the sample, except for a specific section of the chapter.

This example shows the use of `sample` and `full`:

~~~
{sample: true}
# Chapter One

This is included in the sample.

## Section One

This is included in the sample since it is contained in a chapter which is.

{sample: false}
## Section Two

This is *not* included in the sample since it is explicitly excluded, despite
the fact that the chapter is in the sample.

{sample: true}
## Section Three

This is included in the sample. This is redundant since it's in a chapter which
is.

{full: false, sample: true}
# Buy the book!

What you read was just a sample. Why not buy the full book?

# Chapter Two

This has the default values, so it is included in the book or course, but is
excluded from the sample.
~~~

The next example shows the behaviour of `sample`, with and without `cascade`:

~~~
{class: part, sample: true}
# Part One

This introductory text will be included in the sample.

# Chapter One

This chapter is also included in the sample since it is contained in a part
which is in the sample, and since cascade is true by default.

## Section One

This is included in the sample since it is contained in a chapter which is.

{class: part, sample: true, cascade: true}
# Part Two

This introductory text will be included in the sample.

# Chapter One

This chapter is also included in the sample since it is contained in a part
which is in the sample, and since cascade is explicitly set to true.

## Section One

This is included in the sample since it is contained in a chapter which is.

{class: part, sample: true, cascade: false}
# Part Three

This introductory text will also be included in the sample.

# Chapter One

This chapter is NOT included in the sample since the default is false and since
the part which contains it has cascade: false.

## Section One

This section is NOT included in the sample since it is not contained in a
chapter which is.
~~~

To be clear: ALL conditional inclusion attributes **ONLY** have meaning when used
as an attribute list on headings. You can only say `{sample: true}` immediately
above a heading. You can't have a blank line below it (otherwise it would be a
directive, and not be valid) and you can't attach it to anything other than a
heading (like a paragraph, figure, etc.).

Finally, as discussed later, Markua supports "extension attributes", to allow
implementations to add attributes for purposes which are not in the scope of the
Markua spec. One such attribute added by Leanpub is the `{community: true}`
attribute, which functions like a `{sample: true}` attribute to produce a sample
book. However, the `{community: true}` attribute results in Leanpub producing a
sample book which is only available if the reader provides an email address for
the download. This type of feature obviously does not belong in the Markua
spec; the point, however, is that extension attributes make it so easy to add
these types of features that it is almost as though they were in the spec.


### Span attribute lists

Surrounding text in square brackets can be useful not just for giving it a URL
to link to. If you wish to add attributes to an arbitrary span of text, you can
create an arbitrary span of text using square brackets and then add an attribute
list immediately afterward. You can use any attribute list on this span, and you
can also just use the id syntactic sugar `{#theid}` on this span. The most
common uses of this are to add `id` attributes or index entries. (Index entries
are discussed later.)

```````````````````````````````` example
Some text [then a span]{foo: lorem, bar: ipsum} and more text.

This [span has an id]{#hello}, so hooray!

This span [also has an id]{id: world}. 

This span contains [古池や蛙飛び込む水の音]{lang: jpn} Japanese text.
.
<p>Some text <span foo="lorem" bar="ipsum">then a span</span> and more text.</p>
<p>This <span id="hello">span has an id</span>, so hooray!</p>
<p>This span <span id="world">also has an id</span>.</p>
<p>This span contains <span lang="jp">古池や蛙飛び込む水の音</span> Japanese text.</p>
````````````````````````````````

Note, however, that you cannot start a normal span with a caret (`^`): this
creates a `[^footnote]` instead. (Footnotes are discussed below.)



### Extension attributes

Markua Processors may encounter attributes which they do not understand.

Whenever this happens, these attributes must be filtered from the output. A
Markua Processor should function as though there is a whitelist of attributes
which it permits for each element, and filter everything else.

Because of this, Markua attribute lists can contain any number of extension
attributes. An extension attribute is an attribute which is not defined in the
Markua specification. This is true whether the attributes are inserted in an
attribute list attached to a span, block or even in free-floating directives.

Because of the fact that all unrecognized attributes are filtered, it is
possible for a Markua Processor to add extension attributes that only it
understands. This encourages competition in the Markua ecosystem, while ensuring
that Markua implementations do not choke on Markua input which goes beyond their
capabilities.

For example, Leanpub supports an `icon` attribute on blurbs. If a different
Markua processor does not support this attribute, there is no harm done: the
attribute just has no effect.

Extension attributes go far beyond adding icons to blurbs: they allow for
specialized uses of Markua. Since CSS is so powerful, with creative uses of
custom attributes and custom CSS, Markua documents can be transformed. Some
obvious uses of extension attributes include adding CSS classes which can then
be styled to set fonts, etc.

This ensures that new attributes can be added to future versions of Markua
without a negative effect on older Markua implementations. It also ensures that
new versions of Markua can simply stop supporting attributes defined in this
version of Markua without needing to specify anything special.



## Directives (M)

Directives are switches which affect the behavior of a Markua Processor.

The syntax for directives is `{simple}`. A directive is surrounded in curly
braces, and should have a blank line above and below it.

Directives do not have a value. Instead, their presence is all that is
required. Here's the syntax for an imaginary directive `foo`:

~~~
some content

{foo}

some content
~~~

To be clear: a directive does **NOT** contain `{key: value}` pairs. That is an
attribute list, not a directive.

The absence of key-value pairs is what makes a directive a directive. It is a
keyword by itself, enclosed in curly braces on a line by itself. So, the
following is also a directive, albeit a poorly-formatted one:

~~~
some content
{foo}
some content
~~~

The specific directives are discussed next.



### The pagebreak directive

Although authors are encouraged to not think about page breaks, this directive
is a nod to reality: sometimes an author will really, really want to insert a
page break.

This is done with the `{pagebreak}` directive:

~~~
lorem ipsum dolor

{pagebreak}

foo bar baz
~~~

Again, the use of this directive is discouraged. Authors should spend their
time writing, not formatting.

### The mainmatter and backmatter directives

Most published books have three types of material in them: the front matter,
the text (or "main matter") and the back matter. (Yes, this applies to courses
as well as books, since many Markua Processors like Leanpub can generate a
book out of the course material.)

What authors write, the manuscript, is typically what goes into the text,
or main matter, of the book. In style guidelines this is often called the
text; in formats such as TeX and LaTeX it is called main matter. It's what is
typically numbered with Arabic numerals starting from 1. In Markua, the first
page of the mainmatter will be numbered page 1, regardless of whether or not
the page number will be shown on that page.

There's a bunch of other stuff (the Dedication, Epigraph, Table of Contents,
Foreword, Preface, etc.) which can come before the main text of the book. The
stuff before the main matter is called "front matter". The special pages that
can be created by insertion directives are not numbered. The rest of the
frontmatter follows the same page numbering rules as the main matter, except
that numbering uses Roman numerals. The first page of the frontmatter will be
numbered page i, regardless of whether or not the page number will show on that
page.

There's also a bunch of other stuff (appendices, the index, etc.) which can
come after the main matter. This is called the "back matter".

If Markua just relied on its headings support there would be no good way to
accomplish the division of a manuscript into front matter, main matter and
back matter. (We could try some convention about heading names, but that would
be a highly objectionable, English-centric hack.)

So, this is where the mainmatter and backmatter directives come in. With this
approach, your book is essentially the following:

~~~
any frontmatter content, like a dedication, preface or introduction

{mainmatter}

the body of your book (the text or main matter)

{backmatter}

any back matter content, such as appendices
~~~

Note that there is no need for a `{frontmatter}` directive, so it does not
exist. A Markua Processor should ignore it if it is encountered, but a warning
can be provided.

* Everything before the `{mainmatter}` directive is in the front matter.
* Everything after the `{mainmatter}` directive and before a `{backmatter}`
  directive is in the main matter
* Everything after the `{backmatter}` directive is in the back matter

The `{mainmatter}` and `{backmatter}` directives act as dividers, separating a
book or course into logical groups of content.

While these mainmatter and backmatter directives are merely optional hints, there is a
very strict rule about their use: each of them (`{mainmatter}`, `{backmatter}`)
can only appear **once** in a document.

Now, note that the most minimal way to write a book is to use no section
directives at all. With this approach, everything is in the text (main matter)
of the book. Page numbering is in Arabic numerals, etc.

For authors who do not know about the section directive, this is what they are
doing. Nothing bad or unexpected will happen: they can write their book, and it
will look correct. Only when they go to add things like a preface or an
appendix, and feel the need for different numbering, will they need to discover
the section directives. Then, they can add them to their Markua document as
appropriate.

### The insertion directives (dedication, toc, etc.)

With a book or course, certain types of content gets created by the Markua
Processor, or in metadata provided by the author. This content then needs to be
positioned in the book or course.

While a Markua Processor can adopt sensible defaults, sometimes an author wants
more fine-grained control over where this automatically-generated content or
user-provided metadata content is inserted. That's what the various insertion
directives are for: to specify the positioning of where this content is
inserted.

Here's an example book:

~~~
some front matter content

{dedication}

more front matter content

{toc}

{figures}

even more front matter content

{mainmatter}

the main matter content

{backmatter}

some back matter content

{index}

more back matter content

{quiz-answers}
~~~

The following the insertion directives which can **only** occur in the front
matter, which is everything **before** the `{mainmatter}` directive:

* `{half-title}`
* `{series-title}`
* `{title-page}`
* `{copyright}`
* `{dedication}`
* `{epigraph}`
* `{toc}` (for the Table of Contents)
* `{figures}` (for the List of Figures)
* `{tables}` (for the List of Tables)

To be clear, these directives **will not work** if they are inserted **after**
the `{mainmatter}` directive.

Also, note that there is no more `{frontmatter}` directive in Markua. It
existed in Markua 0.10, but it is redundant: front matter is just the stuff
which is comes before a `{mainmatter}` directive. (You should remove it when
migrating a Markua 0.10 document to the version of Markua specified here, but
if you forget, don't worry: Leanpub will ignore the `{frontmatter}` directive,
and other Markua processors should do so as well.)

Note that a Markua Processor can be smart about certain things, if it wants to.
For example, it can choose to not number the pages before the Table of Contents,
but to number the pages after the Table of Contents. Then, the author can use
the `{toc}` directive to choose where in the front matter the Table of Contents
is positioned. This will not only affect its position, it will also affect the
numbering of content before and after it.

The following are the insertion directives which can occur in the back matter,
which is everything after the `{backmatter}` directive:

* `{index}`
* `{exercise-answers}`
* `{quiz-answers}`

The `exercise-answers` and `quiz-answers` directives are used to position
the answers to any exercises or quizzes in the text of the Markua document.

If neither of these directives are present, a Markua Processor should position
any exercise answers somewhere near the back of the book (in the back matter,
if it exists). For quiz answers, on the other hand, a Markua Processor may do
whatever it wants in terms of whether the quiz answers are included in the book,
regardless of the presence or position of the `quiz-answers` directive.

For example, in Leanpub's online courses, the quiz answers are only provided
when quizzes are completed and automatically marked, and are never output in the
material book for the course.

### The lang-* directives

There is one more set of directives, the `lang-*` directives. These all start
with `lang-`.

The `lang-*` directives are important enough that they are discussed in the
next section, which also discusses the `lang` attributie and document setting.



## The lang document setting, lang attribute and lang directives (M)

When generating books, knowing the language being used is very important.
This enables a Markua Processor to do two things:

1. Know which fonts to use (since the fonts to use should take into
   account the language of the text).
2. Know the direction of the text (left-to-right or right-to-left).

Note that the way which default fonts are specified for each language in a
Markua document is not defined in this specification. At a bare minimum, a
Markua Processor should allow users to specify the font to use for each
language.

For example, Leanpub defines standard themes for each supported language,
which specify default fonts to use for titles, body text and code samples.
Leanpub authors can also create custom themes, with their own font choices
for each language, of course. Leanpub authors can even make these custom
themes reusable, so that they can use one theme for all their books.

Anyway, there are three ways for an author to indicate what language is being
used at any given time:

1. The `lang` **document setting**, which sets the default language.
2. The `lang` **attribute**, which temporarily overrides the default language
   for a particular span or block.
3. The `lang-*` **directives**, which set a new default language. This will be
   the default language until another `lang-*` directive is encountered.
   (There is no directive called `lang-*` itself; what this refers to is a
   family of directives like lang-eng, lang-jpn, lang-zh-Hant, etc.)

In all three cases, the value being set is the language code of the
language that the Markua document is written in. This language code is found in
[IS0 639-3](https://en.wikipedia.org/wiki/ISO_639-3), with two exceptions:

1. Traditional Chinese is `zh-Hant`.
2. Simplified Chinese is `zh-Hans`.

Since English is the default language of a Markua document, all Markua
Processors must support the `eng` value. Support for all other language codes
(or `lang-*` directives) is optional. If the language code given as the value
of a `lang-*` directive is not supported or is unrecognized, it must be
interpreted as `eng` and switch the font accordingly and switch the direction
to left-to-right.



### The `lang` document setting

A Markua document has a global `lang` document setting, which sets the default
language. All document settings have default values; the default value of the
`lang` document setting is `eng`.

Document settings are discussed [here](#document-settings-m-).

For a example of the `lang` document setting in action, see the section “A
detailed example” below.



### The `lang` attribute

A `lang` attribute can be applied to a particular block or span.

After that block or span concludes, the lang immediately reverts to the default
language.

~~~
Here is some English text.

{lang: jpn}
古池や蛙飛び込む水の音

{lang: jpn}
春過ぎて夏来にけらし白妙の衣干すてふ天の香具山

This is some paragraph text which defaults to English, and it
contains [古池や蛙飛び込む水の音]{lang: jpn} a span which is in Japanese.

This is some paragraph text which still defaults to English.
~~~

This example used attribute lists on two paragraphs and on a span.

The default language of the document stayed as `eng` throughout, even though
two paragraphs and one span had their language set to Japanese.

Having to specify the `lang` attribute list on every single paragraph which did
not use the default language would get annoying very quickly! This is why the
`lang-*` directives exist.



### The `lang-*` directives

**NOTE FOR LEANPUB AUTHORS: The `lang-*` directives are not yet supported on
Leanpub. We will add support for them sometime in Q1 2024, and remove this
note (TODO) when this is done.**

The `lang-*` directives set a new default language. This will be the default
language until another `lang-*` directive is encountered.

This example shows how to use the `lang-*` directives to temporarily switch the
default language of a Markua document:

~~~
Here is some English text.

The Markua Processor knows it is in English since there is no document settings
block, so the default document setting for `lang` is `eng`.

{lang-jpn}

古池や蛙飛び込む水の音

春過ぎて夏来にけらし白妙の衣干すてふ天の香具山

{lang-eng}

Here is some more English text.
~~~

For another example of the `lang-*` directives, see the section "A detailed
example" below.



### A detailed example

The following example shows a number of uses of the `lang` document setting,
the `lang` attribute, and the `lang-*` directives.

~~~
{
  lang: eng
}

# Chapter One

Note that there was a document settings block at the beginning of this Markua
document. It set the default language to `eng`. Since that is the default of
the `lang` document setting, that was redundant.

This is still in English. Now we will use the `lang-jpn` directive to switch
the default language to Japanese, so that we can output some Haiku
by 松尾芭蕉{lang: jpn} (Matsuo Bashō). (Note that this was a use of the `lang`
attribute on a span).

{lang-jpn}

古池や蛙飛び込む水の音

春過ぎて夏来にけらし白妙の衣干すてふ天の香具山

{lang-eng}

Now we are back in English, since we used the `lang-eng` directive again to
switch the default back to English.

Directives are the correct way to switch the default language for an extended
period of time, but sometimes you may want to just switch the language for a
particular block or span. We already saw how to use the `lang` attribute on a
span; here's how to use it on a block:

{lang: jpn}
夏草や兵どもが夢の跡

Now we are still in English, since the above was a `lang` attribute, not a
`lang-*` directive.

Finally, we'll use the `lang-*` directives again, to switch to Traditional
Chinese and then to Arabic for some more poetry.

{lang-zh-Hant}

花間一壺酒，獨酌無相親。
舉杯邀明月，對影成三人。

{lang-ara}

أَلا كُلُّ شَيْءٍ ما خَلا اللهَ باطِلُ
وَكُلُّ نَعِيمٍ لا مَحالَةَ زائِلُ

{lang-eng}

And with those poems by 李白{lang: zh-Hant} (Li Bai) and
by الْمُتَنَبِّي {lang: ara} (Al-Mutanabbi), we can get back to the spec!
~~~

Implementors of Markua Processers should strongly consider using this example
as a test case!



## Paragraph continuations (M)

In books, you can insert things like lists in the middle of paragraphs.

Ideally, this would be one paragraph. However, it's not:

```````````````````````````````` example
foo
  * lorem
  * ipsum
bar
.
<p>foo</p>
<ul>
<li>lorem</li>
<li>ipsum
bar</li>
</ul>
````````````````````````````````

Here, you lose hard, because "bar" is considered to be part of the same list
item as ipsum. I think this is a mistake, but it is more important to preserve
Markdown compatibility than to fix this mistake.

Ideally, it would also be possible to insert a list inside a paragraph.
However, it's not:

```````````````````````````````` example
foo

  * lorem
  * ipsum

bar
.
<p>foo</p>
<ul>
<li>lorem</li>
<li>ipsum</li>
</ul>
<p>bar</p>
````````````````````````````````

The reason here is that HTML does not support inserting lists in paragraphs.
Lists are always siblings to paragraphs, not contained within them.

There is no way to fix this at the HTML tag level, since this is just how HTML
works. It would be possible to just construct a fake list with a bunch of CSS
and break tags inside a paragraph, but that would be far worse.

From an HTML perspective, you cannot insert lists inside paragraphs. But if
you are writing in Markua, you're not just writing HTML. You can also be
generating a book in PDF and other non-HTML formats, which do support lists
inside paragraphs just fine.

So, there needs to be some way to indicate that the beginning of a paragraph
after something like a list is actually a continuation of the previous
paragraph.

Now, this could have been done by just not adding blank lines, only single
newlines, but that would involve fighting against existing Markdown behavior.
Worse, it would introduce an incompatibility just to fix something that many
people don't consider broken.

For authors who want a list to appear to be inside a paragraph, Markua does have
two solutions. First, you can just add a `continued-para` class to the
subsequent paragraph, via an attribute list:

```````````````````````````````` example
foo

  * lorem
  * ipsum

{class: continued-para}
bar
.
<p>foo</p>
<ul>
<li>lorem</li>
<li>ipsum</li>
</ul>
<p class="continued-para">bar</p>
````````````````````````````````

It would then be up to the Markua Processor to use the appropriate CSS to make
the paragraph with the `continued-para` class look like a continuation of the
previous paragraph, such as by removing indentation or decreasing leading space.

(Attribute lists, discussed later, are one of the main contributions of Markua.)

However, if you are an author who cares about this feature, typing an attribute
list whenever you want to continue a paragraph is really obnoxious. So, there
needs to be some syntactic sugar, and there is, with the `^` symbol.

A caret (`^`) on a line by itself above a paragraph functions as a
`{class: continued-para}` attribute list for the following paragraph:

```````````````````````````````` example
foo

  * lorem
  * ipsum

^
bar
.
<p>foo</p>
<ul>
<li>lorem</li>
<li>ipsum</li>
</ul>
<p class="continued-para">bar</p>
````````````````````````````````


Note that this works for more than just lists. In fact, it has nothing to do
with lists, or even with images or other resources. It's all about the
indentation behavior of a paragraph, typically following some other block. So,
you can use this whenever you want:

```````````````````````````````` example
foo

> a block quote

^
bar
.
<p>foo</p>
<blockquote>
<p>a block quote</p>
</blockquote>
<p class="continued-para">bar</p>
````````````````````````````````



## Concatenating manuscript files inserts blank lines between them (M)

A Markua document can be written in one file or multiple manuscript files. If a
manuscript is written in multiple files, these files are concatenated together
by the Markua Processor to produce one temporary manuscript file, and that one
file is used as the input.

The way that the list of files that should be included in the book or course
being generated from a Markua manuscript is defined is implementation-specific,
and is not formally specified.

For example, Leanpub enables authors to write books and courses either in a web
browser or in local files on an author's computer. In the browser mode, the
author chooses a file to edit via a web UI, which also allows the files to be
reordered, etc. In the local files mode, the author creates a file called
`Book.txt` which just lists the files in order. The reason that this is worth
mentioning is that in both cases, what results is a simple list of files, and
nothing more. All semantics about which content is included in, say, the full
book or the sample book is determined by attribute lists, which are specified
in the Markua specification.

Now, even though the way that the list of files is defined is not part of the
Markua spec, the way that the files are concatenated together to produce the
full manuscript *is* part of the Markua spec.

Importantly, in order to avoid a number of bugs, the files are not just
concatenated together unchanged--they **must** be concatenated together by
Markua Processors by adding **two newlines** (i.e. one blank line) between the
end of each file and the beginning of the next file.

So, after this process, exactly one blank line separates the contents of each
manuscript file. Note that because of this rule, a paragraph (or any other block
element) cannot span multiple manuscript files.

To see why this approach is so important, consider the following single-file
Markua document:

```````````````````````````````` example
# Chapter One

Lorem ipsum dolor.

# Chapter Two

Yada yada yada.
.
<h1 class="chapter">Chapter One</h1>
<p>Lorem ipsum dolor.</p>
<h1 class="chapter">Chapter Two</h1>
<p>Yada yada yada.</p>
````````````````````````````````

Suppose instead a multiple-file approach was used, in which there were two files,
ch1.txt and ch2.txt, with the following content.

ch1.txt:

```````````````````````````````` example
# Chapter One

Lorem ipsum dolor.
.
<h1 class="chapter">Chapter One</h1>
<p>Lorem ipsum dolor.</p>
````````````````````````````````

ch2.txt:

```````````````````````````````` example
# Chapter Two

Yada yada yada.
.
<h1 class="chapter">Chapter Two</h1>
<p>Yada yada yada.</p>
````````````````````````````````

If Markua did not add any newlines between files, then these files would produce
the following incorrect manuscript:

```````````````````````````````` example
# Chapter One

Lorem ipsum dolor.# Chapter Two

Yada yada yada.
.
<h1 class="chapter">Chapter One</h1>
<p>Lorem ipsum dolor.#Chapter Two</p>
<p>Yada yada yada.</p>
````````````````````````````````

If Markua only added one newline when concatenating, this would produce a correct
manuscript (since headings end paragraphs), but one with possible bugs:

```````````````````````````````` example
# Chapter One

Lorem ipsum dolor.
# Chapter Two

Yada yada yada.
.
<h1 class="chapter">Chapter One</h1>
<p>Lorem ipsum dolor.</p>
<h1 class="chapter">Chapter Two</h1>
<p>Yada yada yada.</p>
````````````````````````````````

Worse, since a number of text editors such as Emacs have a "strip blank lines at
the end of files" setting, it would be possible to introduce such a bug if Markua
simply relied on blank lines being added to the end of a file by the author.

So, because of the blank line rule, concatenating the files produces the same
manuscript as the single-file manuscript above:

```````````````````````````````` example
# Chapter One

Lorem ipsum dolor.

# Chapter Two

Yada yada yada.
.
<h1 class="chapter">Chapter One</h1>
<p>Lorem ipsum dolor.</p>
<h1 class="chapter">Chapter Two</h1>
<p>Yada yada yada.</p>
````````````````````````````````

</div>



<div class="extension">

# Document settings (M)

Markua's philosophy is that non-semantic formatting is procrastination. Because
of this, almost all formatting is done by global switches. These switches are
called document settings, and they are defined once, at the beginning of the
document. Document settings affect the formatting of the entire book or course.

Document settings are defined in one or more document settings hashes. The
document settings hash or hashes must be at the start of a Markua document.
Once something other than a document settings hash or a blank line is
encountered, no more document settings hashes can be defined. (Document
settings hashes can be separated with single or multiple newlines, but once
something other than a document settings hash or blank line is encountered,
no more document settings hashes can be defined.)



## Document settings are specified in simple key-value pairs or in JSON (M)

There are two ways to specify a document settings hash:

1. As a newline-separated flat list of key-value pairs (`key: value`).
   With this format, every non-blank line in the document settings hash must
   contain a key, a colon `:`, and a value. Any whitespace at the beginning
   or end of the keys and values will be stripped.
2. As [JSON](https://www.json.org/json-en.html).

The Markua Spec only defines a short list of the standard document settings
which all Markua Processors must support. In the spec, these settings are
defined in a flat list of `key: value` pairs, but they can also be specified
using JSON.



## The standard document settings (M)

The Markua Spec only defines the settings which all Markua Processors **MUST**
support. It is expected that Markua Processors will define their own
implementation-specific settings somehow, either as document settings which
are in a flat list, as JSON, or both.

Since Markua defines a number of document settings, it is helpful to see what
the defaults are in one list. These defaults are listed below.

```
{
  alt-title: all
  chapter-number-format: name
  default-code-language: guess
  figure-classes: all
  include-page-numbers-in-references: true
  italicize-underlines: true
  lang: eng
  list-figures: combined
  number-figures: sequential
  part-number-format: name
  restart-endnote-numbering: true
  restart-footnote-numbering: true
  section-number-format: name
  soft-breaks: space
  toc-chapter-number-format: name
  toc-part-number-format: name
  toc-section-number-format: name
  two-space-hack: true
}
```

What these settings all mean is explained in the rest of this section.

Note that the behavior of resources which are treated as figures is controlled
by four related settings: `alt-title`, `figure-classes`, `list-figures` and
`number-figures`. These need to be considered together: the behavior of these
settings are tightly coupled.

`alt-title`
: `all`, `text` or `none`. The default is `all`, for compatibility with Markua
0.10 and Leanpub Flavoured Markdown by default. This setting controls whether,
in the cases where there is no figure title explicitly set, the alt text on a
resource is removed and used as the figure title instead. This setting is so
important it is discussed in its own [section](#the-alt-title-document-setting-m-)
below.

`chapter-number-format`
: `category_number_name` (e.g. Chapter 1: The Chapter Name), `category_number`
(e.g. Chapter 1), `number_name` (e.g. 1 The Chapter Name), `number` (e.g. 1)
or `name`  (e.g. The Chapter Name). The default is `name`. This determines the
format of the chapter heading in the body of the book. The format of the
chapter heading in the table of contents is set by the related
`toc-chapter-number-format` setting.

`default-code-language`
: The default language that code which is a local resource, web resource or
inline resource inserted as a **block** with three backticks is interpreted as.
The default value is `guess`, which means to guess at the code language based on
the syntax encountered (or the file extension for external code samples), and
attempt to syntax highlight appropriately. A good alternative is `text`, which
means no syntax highlighting should be used, but the code should be in a
monospaced font suitable for a programming language. Besides these options, you
can specify a particular programming language used, such as `ruby` or `java`.
If a Markua Processor does not recognize the programming language specified,
it must format it as `text`. Finally, note that the value of this setting only
affects local resources, web resources or inline resources inserted as a block
with three backticks--it has no effect on code spans, or on inline resources
inserted with tildes. The default language for inline resources delimited by
three tildes is always `text`, but you can override the default on individual
resources of course. Finally, code spans do not support syntax highlighting, so
there is no equivalent setting for the default language of code spans.

`figure-classes`
: this is a comma-separated list of the figure classes which are included in the
list(s) of figures near the beginning of the book or course. The default is
`all`, which means that all figures are included, regardless of the value
of their `class` attribute. An example of this setting is:
`figure-classes: equation, figure, image, listing`. Note that whether the
figures are listed, either grouped by their class or together in one big list,
is controlled by the `list-figures` setting. Note that the numbering of figures
is controlled by the `number-figures` setting.

`include-page-numbers-in-references`
: `true` or `false`. The default value is `true`. When a smart crosslink to a
figure reference is inserted with `#f`, this setting determines how the
reference is displayed. If true (the default), the text includes the page
number (e.g. "Figure 8.2: Anatomy of a Squirrel on page 42") when inserted in
a PDF; if false, it does not (e.g. "Figure 8.2: Anatomy of a Squirrel". This
setting has no effect on EPUB, which never includes page numbers in references
(since the text is resizable).

`italicize-underlines`
: `true` or `false`. The default value is `true`. If `italicize-underlines` is
`true`, then `_this_` and `*this*` are both italic. If `italicize-underlines` is
`false`, then `_this_` is underlined while `*this*` is italic. This is discussed
[here](#underline-m-).

`lang`
: The [IS0 639-3](https://en.wikipedia.org/wiki/ISO_639-3) three character
language code of the language that the Markua document is written in. The
default is `eng`.

`list-figures`
: `combined`, `groups` or `none`. The default value is `combined`. This setting
controls whether figures are listed in one or more lists near the beginning of
the book. Note that whether these figures are numbered is controlled by the
`number-figures` setting. The `list-figures` setting respects the numbering
decision specified there. This is what the values mean:

* `none`: No lists of figures are produced.
* `combined`: One combined list of figures is produced for all figures in the
  manuscript, ordered by their appearance in the manuscript. This list is called
  List of Figures for English books, and localized into the language of the book
  or course. This produces figures named Figure, and (by default) numbered
  sequentially: Figure 1, Figure 2, Figure 3, etc. The numbering is defined by
  the `number-figures` document setting, specified below.
* `groups`: The Markua Processor should group figures by their classes or types.
  Here's how the grouping works:
  1. If there is no `class` attribute, the figure is grouped in a list with other
     resources of its `type`. This is a nice default, since it results in a List
     of Tables, List of Code Samples, List of Images, etc. The type of a
     resource can be specified by the type attribute, or inferred from the
     format of the resource.
  2. If there is a `class` attribute, this overrides the type. For example, it
     is almost never a good idea to have a List of Verbatim, so if you add a
     `class` of `poem` you will get a List of Poems. (There is nothing special
     about `poem`, by the way: if you add a `class` of `sonnet`, you will get a
     List of Sonnets. This is not just useful to differentiate types of poetry,
     by the way: this approach can be used to automatically produce a
     List of Lemmas, List of Theorems, etc.

The combination of `list-figures: groups` and `number-figures: chapter`
produces figures like Listing 1.1, Listing 1.2, Listing 4.1 and Image 1.1,
Image 1.2, Image 2.1, Image 2.2, Image 4.1, etc. This is a good combination of
document settings for books like computer programming books.

`number-figures`
: `chapter`, `none`, or `sequential`. The default is `sequential`. If the value
is `chapter` or `sequential`, then resources are numbered when they are inserted
as figures. If the value is `sequential`, you get numbering like `Figure 1`,
`Figure 2`, `Figure 3`, etc. If the value is `chapter`, you get numbering like
`Figure 1.1`, `Figure 1.2`, `Figure 4.1`, etc. If the value is `none`, figures
are not numbered.

`part-number-format`
: `category_number_name` (e.g. Part 1: The Part Name), `category_number`
(e.g. Part 1), `number_name` (e.g. 1 The Part Name), `number` (e.g. 1)
or `name`  (e.g. The Part Name). The default is `name`. This determines the
format of the part heading in the body of the book. The format of the
part heading in the table of contents is set by the related
`toc-part-number-format` setting.

`restart-endnote-numbering`
: `true` or `false`. Default `true`. Whether the endnote numbering is restarted
at the end of a chapter.

`restart-footnote-numbering`
: `true` or `false`. Default `true`. Whether the footnote numbering is restarted
at the end of a chapter.

`section-number-format`
: `number_name` (e.g. 1.1 The Section Name), `number` (e.g. 1.1) or `name`
(e.g. The Section Name). The default is `name`. This determines the format of
the section heading in the body of the book. The format of the section heading
in the table of contents is set by the related `toc-section-number-format`
setting.

`soft-breaks`
: `break` or `space`. The default is `space`, for compatibility with
Markdown. This setting determines the behavior of single newlines (i.e. a
newline which is not followed by another newline) in the output. With `space`,
single newlines in the source essentially produce a single space in the output
(like Markdown and Leanpub Flavoured Markdown). With `break`, single newlines
inside paragraphs in the Markua document essentially produce newlines in the
output (like Markua 0.10). This setting is so important it is discussed in its
own [section](#the-soft-breaks-document-setting-m-) below.

`toc-chapter-number-format`
: `category_number_name` (e.g. Chapter 1: The Chapter Name), `category_number`
(e.g. Chapter 1), `number_name` (e.g. 1. The Chapter Name), `number` (e.g. 1)
or `name`  (e.g. The Chapter Name). The default is `name`. This determines the
format of the chapter heading in the table of contents of the book. The format
of the chapter heading in the body is set by the related
`chapter-number-format` setting.

`toc-part-number-format`
: `category_number_name` (e.g. Part I: The Part Name), `category_number`
(e.g. Part 1), `number_name` (e.g. I. The Part Name), `number` (e.g. I)
or `name`  (e.g. The Part Name). The default is `name`. This determines the
format of the part heading in the body of the book. The format of the
part heading in the table of contents is set by the related
`toc-part-number-format` setting.

`toc-section-number-format`
: `number_name` (e.g. 1.1 The Section Name), `number` (e.g. 1.1) or `name`
(e.g. The Section Name). The default is `name`. This determines the format of
the section heading in the body of the book. The format of the section heading
in the table of contents is set by the related `toc-section-number-format`
setting.

`two-space-hack`
: `true` or `false`. The default is `true`, for compatibility with Markdown.
  If `true`, adding two spaces at the end of a line forces a hard line break.
  If you use this setting, ensure that your text editor does not strip
  trailing whitespace! This is probably the best choice if `soft-breaks` is
  set to `space`, or if you want maximum compatibility with Markdown.
  If `false`, spaces at the end of a line are ignored, and the single newline
  is treated as a single newline would be without those spaces, which is
  determined by the value of the `soft-breaks` setting. Since Markua allows
  the behavior of soft line breaks to be configured with the `soft-breaks`
  setting, the `two-space-hack` is not needed if the `soft-breaks` setting
  is `break`. However, even in this case, authors may wish for the two space
  hack to still function as it does in Markdown, in order to support existing
  Markdown text or in cases where there are multiple authors of a manuscript.


## The soft-breaks document setting (M)

As discussed above, the `soft-breaks` is one of the document settings that a
Markua document can have. (Document settings are set at the beginning of a
Markua document, or via a user interface such as a web page.)

The `soft-breaks` document setting can be `break` or `space`. The default is
`space`, for compatibility with Markdown.

**If you want the behavior of Leanpub Flavoured Markdown, you need to choose
`space`.**

**If you want the behavior of Markua 0.10, you need to choose `break`.**

This setting determines the behavior of single newlines (i.e. a
newline which is not followed by another newline) in the output. This is what
the values mean:

`space`
: Single newlines inside paragraphs in the Markua source produce
  output which functions like a single space. In output such as PDF, a single
  space must be what is output. In output such as HTML, a single newline
  is output, since in HTML output, a single newline is equivalent to a single
  space in terms of the effect on presentation. So, this type of line break
  only is a line break in the HTML source, and in all output presentation it
  functions like a space--hence the name `space`. The `space` option is the
  default for compatibility with Markdown by default. If you want the
  behavior of Markua 0.10, choose `break`.

`break`
: Single newlines inside paragraphs in the Markua document produce
  break tags in the HTML output and line breaks in non-HTML output like
  LaTeX or PDF. The reason this value is called `break` is to indicate that
  when you hit a newline to get a line break, you get a real line break.
  To be clear: the `break` attribute means to insert a forced linebreak
  **whenever a single newline is encountered in the manuscript, regardless
  of whether there are two spaces at the end of the line**. While this makes
  some intuitive sense, it is very much against the default behavior of
  Markdown.  The `break` option gives you the behavior of Markua 0.10,
  in which all single newlines function as though they were using the
  "two space hack" to produce a hard line break in normal Markdown. (Whether
  the "two space hack" is enabled in a Markua document is determined by the
  value of the `two-space-hack` setting, discussed below.)


## The alt-title document setting (M)

As discussed above, the `alt-title` is one of the document settings that a
Markua document can have. (Document settings are set at the beginning of a
Markua document, or via a user interface such as a web page.)

The `alt-title` document setting controls whether, in the cases where there is
no figure title explicitly set, the alt text on a resource is used as the
figure title instead of as alt text.

The `alt-title` document setting can have the value of `all`, `text` or `none`.
The default is `all`, for compatibility with Markua 0.10 and Leanpub Flavoured
Markdown by default. These are the three choices, and what their values mean:

* `all`: if no title is provided for a given resource, regardless of the
resource type, remove the alt text and use it as the figure title instead. (This
choice is the default, since it is the closest to the behavior of LFM and of
Markua version 0.10. So, regardless of whether you like the choice, there are 12
years worth of books on Leanpub which would be broken by a different default.)
* `text`: if no title is provided for a given text-based resource (such as
an external code sample), remove the alt text and use it as the figure title
instead. However, if the resource is not purely text-based, such as an image or
a video, do not remove the alt text to use it as the figure title, even if
no figure title is provided. This is arguably the most sensible choice, as it
strikes a balance between preserving compatibility with Markdown for
non-text-based resources like images, while still providing a sensible default
for text-based resources like code samples. Since Markdown does not define
support for external text-based resources, there is no conflict here.
* `none`: never remove the alt text to use it as the figure title, even for a
text-based resource where no figure title is provided. (This is how Markdown
behaves.)

Using the alt text as the figure title is the most terse syntax, and may make
sense if no alt text is needed. For example, valid use cases include:

* Code samples and other text-based resources. Since these resources are
  themselves text, it makes no sense to have different alt text.
* Images, if the only output target is a format like print-ready PDF where
  no alt text is needed.

If alt text is used as the figure title, it is NOT also provided as alt text,
since it would be very annoying to hear the same text spoken twice (once as the
alt text and once as the figure title) when using a screen reader.

Now, it is a debatable point whether using alt text as figure captions is a
good idea. The main argument in favour of it is terseness: it is the most terse
syntax, and the easiest to remember.

A second argument is that in some cases such as external code samples and other
text-based resources, there is no conceivable use for alt text, so the way to
add a title with the least syntax is to use the spot for the alt text.

A third argument in favour of it is aesthetics, since the alternative syntax
in which a title is provided without any alt text looks like this:

```
![](someimage.png "A Figure Caption")
```

With this syntax, you have the `[]` for what looks like no real reason.

Now, the main argument against using alt text as figure captions is very
straightforward: you are supposed to actually provide alt text, and the alt
text should be different from the figure caption.

Alt text is supposed to be supposed to be descriptive (say to provide a literal
description of an image for people with visual disabilities, or who are
listening to the book as an audiobook), whereas good figure captions are terse
(and often witty). These are different requirements, so they should have
different text.

With this argument in mind, if it looks like a bit of an error to have empty
square brackets, that's a good thing: according to this argument, you should
always provide alt text, and it should be different from the figure caption.
Having missing alt text look like a bit of a mistake is actually correct.

Now, this is a very good argument, but as shown above, there are good
counterarguments as well. So, the behavior of the alt text is determined by the
`alt-title` document setting, and the default value is `all` (not `text` or
`none`). The reason why the default is `all` is simple: there are over a decade
of Leanpub books which do this, since they were written in Leanpub Flavoured
Markdown or Markua 0.10:

```
![A Figure Caption](someimage.png)
```

Despite what one's opinion of what the correct value of `alt-title` should be,
the default value of `alt-title` is an acknowledgment of Markua's history.

So, even though the default value of `alt-title` is `all`, the conclusion
I've come to is that the default behavior of Markdown is best for image
resources, and alt text should be used for descriptions, not figure captions,
in the case of images. If you're in this camp, the way to use alt
text, and the best syntax when inserting an image, is to do the following:

```
![some descriptive alt text](someimage.png "A Figure Caption")
```

Note that you can use that syntax even with the default `alt-title: all`,
since the explictly-defined title will take precedence over the alt text.
The `alt-title` document setting only defines what to do when there is no
title set.

To be clear, the `alt-title` document setting never makes a figure caption
be used as the alt text, and it makes alt text be used as a figure caption
if and only if no title attribute is provided, either in quotes or in the
attribute list.

In these examples, the value of the `alt-title` document setting does not matter:

```
![This is always alt text, since a title is provided](someimage.png "A Figure Caption")

{title: "A Figure Caption"}
![This is always alt text, if a caption is provided](someimage.png)

{title: "This is ALWAYS a Figure Caption, Regardless of alt-title"}
![](someimage.png)

![](someimage.png "This is ALWAYS a Figure Caption, Regardless of alt-title")
```

Again, to be clear, the only thing that the `alt-title` document setting
affects is whether the alt text is used as the figure caption instead of as alt
text if there is no title attribute provided as the figure caption.

In these examples, the value of the `alt-title` document setting does matter:

```
![No title, so this will be the figure caption if alt-title is all](someimage.png)

![This will NOT be the figure caption if alt-title is text or none](someimage.png)
```

## Multiple document settings hashes are permitted (M)

A Markua document can define multiple document settings hashes.

There must be at least one blank line between the end of the settings block and
the start of any other document content other than another document settings
hash.

The reason that more than one document settings hash is permitted is that a
Markua Processor may also get settings from elsewhere, like a web interface.
The most straightforward thing to do in such a case is to just add another
document settings hash at the beginning of the file, and then let the Markua
Processor sort out any conflicts.

In the case of a conflict between settings defined in a document and settings
defined, say, in a web interface, the last setting definition must win.
However, a Markua Processor may override settings whose values are unsupported
or illegal.

Here's an example of a Markua document which could be generated if there is a
settings block produced by a web interface, and then a settings block manually
added at the start of a book:

~~~
{
  alt-title: all
  figure-classes: equation, figure, image, listing
  list-figures: groups
  number-figures: chapter
  soft-breaks: space
  two-space-hack: false
}

{
  italicize-underlines: true
}

# Chapter One

Lorem ipsum dolor...
~~~

Now, most tools will not output their settings as key-value pairs, but will
instead output them as JSON. JSON is easy to output and to parse, since there
are good JSON libraries for almost all popular programming languages.

So, here's the same example, but with the automatically-generated set of
document settings specified as JSON:

~~~
{
  "alt-title": "all",
  "figure-classes": [
    "equation",
    "figure",
    "image",
    "listing"
  ],
  "list-figures": "groups",
  "number-figures": chapter,
  "soft-breaks": "space",
  "two-space-hack": false
}

{
  italicize-underlines: true
}

# Chapter One

Lorem ipsum dolor...
~~~

Regardless of how the automatically-generated document settings are output,
the author will typically only see the manually-defined document settings
block. So, the formatting of the automatically-generated document settings
doesn't really matter, and JSON is the pragmatic choice.



## Custom document settings are encouraged (M)

Finally, Markua Processors can also add their own document settings, just as
Markua Processors can understand their own extension attributes.

For these custom document settings, JSON is ideal, since JSON supports nested
objects, arrays, and all the data types which are needed.

It is expected that Markua Processors such as Leanpub will add a bunch of their
own custom document settings, typically in JSON, in order to control the
formatting of all parts of a Markua document. This is encouraged.

It is deliberate that these document settings are not being formally-specified:
doing so would actually serve to limit innovation. There are many ways to
format a document, and Markua should not seek to define the one true way to do
so.
</div>



<div class="extension">

# Resources (M)

Markua documents are written in plain text, either in one text file or multiple
text files. However, modern books and courses sometimes contain more than text.
Books and courses may embed many kinds of [resources](@).

Resources vary in four different ways:

1. Type
2. Format
3. Class
4. Location

While resources are a very important addition that Markua makes to Markdown,
they are exactly that: an addition. Resources provide an important conceptual
framework, while remaining purely additive to standard Markdown. Specifically,
all images, fenced code blocks and tables supported by CommonMark or GFM work in
Markua unchanged.

What resources do is provide consistency, both in the conceptual understanding,
in adding support for attribute lists, and specifying standard `type`, `format`
and `class` attributes for all resources, including the resources such as
images, fenced code blocks and tables which are also found in other Markdown
dialects.

We have already discussed attribute lists earlier for things like part headings
and conditional inclusion attributes, but by far the most important use of
attribute lists in Markua is with resources. For resources, attribute lists can
be used to specify a figure title and alt text, set an id, specify the format,
and do other useful things.

While resources are an important addition that Markua makes to Markdown, they
were not created in a vacuum. As you'll see, the syntax for local and web
resources is similar to Markdown's inline image insertion syntax, and the syntax
for inline resources is just the fenced code blocks syntax from CommonMark with
the addition of format specifiers and attribute lists. Furthermore, the standard
Markdown image syntax is the inspiration for the syntax of all local and web
resources.

Before we consider how resources vary by type, format, class and location, we
will consider what they have in common. Specifically, resources share the
following things:

1. Attributes and attribute list support
2. Behavior as a figure or a span based on the presence of title
3. Support for a title to be provided, either in the attribute list, in quotes
   or from alt text based on the `alt-title` document setting.



## Resource attributes (M)

Resources have attributes. Attributes can be specified in two ways:

1. In attribute lists.
2. In the syntax for the resource insertion.

As you will see soon, resources can be local, web or inline.

The syntax for a local or web resource inserted as a figure is as follows:

~~~
{key: value, comma: separated, optional: attribute_list}
![optional alt text](resource_path_or_url "Optional Figure Title")
~~~

The syntax for an inline resource inserted as a figure is as follows:

~~~
{key: value, comma: separated, optional: attribute_list}
```optional_format
inline resource content (default format is `guess` with backticks)
```
~~~

You can also insert an inline resource using three or more tildes (`~`) as the
delimiter, instead of the more typical backticks (`):

```
{key: value, comma: separated, optional: attribute_list}
~~~optional_format
inline resource content (default format is `text` with tildes)
~~~
```

To be clear, the number of backticks or tildes can be three or more. This is
just the fenced code blocks syntax, which is discussed later.

Finally, a local or web resource can also have attributes specified after the
resource itself. This is a lot less common, and is discouraged, but it is
documented for completeness:

~~~
![optional alt text](resource_path_or_url "Optional Figure Title"){key: value, comma: separated, optional: attribute_list}
~~~

This is gross, but the reason it is supported is since resources can be inserted
in a sentence, and attribute lists (e.g. for index entries) can be added this
way.

### The attribute list takes precedence

It is always an error to specify an attribute both in the attribute list for a
resource and in the syntactic sugar locations, either after the backticks or in
the square brackets.

However, if this is done, then the value in the attribute list takes precedence,
and the Markua Processor should report the error to the author in a list of
errors and warnings. (Errors and warnings must never be output by a Markua
Processor in the book or course itself, but must always presented in a separate
list for the author's use only.)

In the following figure, the format is `text` not `ruby`:

~~~
{format: text}
```ruby
puts "hello world"
```
~~~

In the following figure, the alt text would be "foo" not "bar":

~~~
{alt: foo}
![bar](foo.png)
~~~

In the following figure, the title text would be "foo" not "bar":

~~~
{title: foo}
![](foo.png "bar")
~~~

Again, the Markua Processor should treat all of these as errors, and output them
in a list of errors and warnings.



## Common resource attributes (M)

The supported attributes vary based on the type of resource, but **all**
resources support the `class`, `format`, `title` and `type` attributes.

As you recall, resources vary in four ways:

1. Type
2. Format
3. Class
4. Location

The first three of these ways that resources vary are specified via the
`type`, `format` and `class` attributes. These attributes are discussed briefly
here, and in the sections below. The fourth way, location, is discussed in a
later section below.

These are the attributes that resources all share:

`class`
: All attribute lists, including those for resources, support a `class`
attribute. When used on a resource, this is the class of the resource. This can
be used for styling, and it can also be used by Markua Processors which group
figures by classes. **Note that there is no need to set a `class` of `figure`
on a resource. That would be too verbose and error-prone. Instead, all
resources with titles are treated as figures.** This is discussed extensively
later.

`format`
: This is the resource format. Different resource types have different legal
values for format. Specific resource types and formats are discussed throughout
the rest of the spec.

`title`
: If a resource has a title, it is a figure. This is discussed in the next
section.

`type`
: This is the resource type. This is usually inferred from the format instead of
being specified.



## Resource titles make figures (M)

All resources with `title` attributes are figures.

This is true regardless of whether the title is specified in quotes, in an
attribute list, or in the alt text (with the appropriate value of the
`alt-title` document setting). In all cases, the resource is a figure.

To be clear, if `alt-title` is `all` (the default), the following three image
resources are all figures, and should look identical:

~~~
![](mac.jpg "The Original Mac")

![The Original Mac](mac.jpg)

{title: "The Original Mac"}
![](mac.jpg)
~~~

The title is text which is shown near the figure, typically above or below it.
A Markua Processor can choose where to position titles based on any criteria it
chooses. For example, a Markua Processor can position all titles above or below
figures, or use a different behavior based on the resource format (e.g. table
titles above, image titles below).

Note that the figure title itself may contain Markua text formatting (e.g. bold,
italic). This text is also displayed for the figure wherever the figure is
listed (e.g. List of Illustrations, Table of Tables, etc.).

The title of a resource shows up in two places in the output:

1. Near the resource, typically above or below it, per the preference of the
Markua Processor.
2. Based on the resource type, either in the List of Illustrations, List of
Tables or Table of Figures, if they are generated for the book. This text
should also be a crosslink to the title inserted near the figure itself.

The title of a resource used as a figure can be specified in three ways:

1. In an attribute list:
   ~~~
   {title: "My Amazing Algorithm"}
   ![](algorithm.rb)
   ~~~
2. In quotes after the filename or URL of a local or web resource:
   `![](resource_path_or_url "Optional Figure Title")`
3. In the alt text, assuming an appropriate value of the `alt-title` document
   setting. The `alt-title` document setting is discussed
   [in great detail here](#the-alt-title-document-setting-m-).

Since inline resources do not use the bracket syntax, any titles must be added
in the attribute list:

~~~
{title: "My Amazing Algorithm"}
```ruby
puts "hello world"
```
~~~

Note that an attribute list can also be provided after the resource, but this is
discouraged since it is less readable:

```
![](algorithm.rb){title: "My Amazing Algorithm"}
```

If a resource has a title (provided by any of the above approaches), it is a
figure.

If a resource does not have a title, it is inserted as a span. This is useful
for images being added like an emoji, for example:

```
Here's a ![poop](poop.png) emoji.
```

Note that in this case, there is already a poop emoji, so this is wasteful as
the emoji syntax can just be used instead. However, if you wanted to use an
image of your favourite politician as a poop emoji, you wouldn't have an emoji
for that person built in, so this syntax would work in its stead.

Again, if a title was added, things would be different:

```
Here's a ![poop](poop.png "The Year 2022 So Far") emoji.
```

In this case, the resource would be a figure, and it would flow in the text as
a figure.

But what if we wanted to specify image alignment for this resource? This could
be done with an attribute list like this:

```
Here's a ![poop](poop.png "The Year 2022 So Far"){align: left} emoji.
```

Since the image resource has a title, it is being inserted as a figure, and it
would flow in the layout like any other figure.

Specifically, this would be equivalent to the following:

```
Here's a
![poop](poop.png "The Year 2022 So Far"){align: left}
emoji.
```

This would also be equivalent to the following:

```
Here's a

![poop](poop.png "The Year 2022 So Far"){align: left}

{class: continued-para}
emoji.
```

This would also be equivalent to the following:

```
Here's a

![poop](poop.png "The Year 2022 So Far"){align: left}

^
emoji.
```

**To be clear: inserting a figure inline in a sentence is equivalent to adding
a single line break before and after it, which is also equivalent to adding a
blank line before and after it with a `class` of `continued-para` on the
paragraph after it, which is also equivalent to the syntactic sugar equivalent
with a caret `^` on the paragraph after it.**

However, this example is both contrived and terrible.

A much better way to write something like this is as follows, using the
attribute list in its typical place above the resource:

```
Here's an emoji.

{align: left}
![poop](poop.png "The Year 2022 So Far")

Let's hope the year improves.
```

This is why almost all figures are added this way.

But just because that's the convention, it does not make it the rule. That is
semantically identical to the following:

```
Here's an emoji.

![poop](poop.png "The Year 2022 So Far"){align: left}

Let's hope the year improves.
```

Finally, if you are implementing a Markua Processor and are trying to determine
the `class` of a resource, the following pseudocode algorithm can help:

```
What's the `class`?

if the class was explicitly set to something
  that's the class
elsif there is a title
  the class is `figure`
else
  the class is undefined
end
```

A resource with an undefined class has no title and is not a figure.

## Resource figure numbering (M)

**NOTE FOR LEANPUB AUTHORS: The `numbered` attribute is not implemented in
Leanpub yet. We plan to add this in 2024, after which point this
note will be removed.**

When a resource has a title, it is a figure.

By default, figures are numbered. This numbering is controlled by the
`number-figures` document setting.

`number-figures`
: `chapter`, `none`, or `sequential`. The default is `sequential`. If the value
is `chapter` or `sequential`, then resources are numbered when they are inserted
as figures. If the value is `sequential`, you get numbering like `Figure 1`,
`Figure 2`, `Figure 3`, etc. If the value is `chapter`, you get numbering like
`Figure 1.1`, `Figure 1.2`, `Figure 4.1`, etc. If the value is `none`, figures
are not numbered.

If a resource does not have a title, it is not a figure, and it is not
numbered.

If you do not want to number any of your figures, you can use the `none`
setting of the `number-figures` document setting to have none of them
numbered.

However, sometimes you may wish to number most of your figures, but not all of
them. In other words, you may wish to give a resource a title (making it a
figure), but not have that resource be numbered like a figure.

To do this, you can use the `numbered: false` attribute.

`numbered`
: `true` or `false`. The default is `true`. This controls whether a particular
resource is numbered. Note that if `number-figures` is `none` then no resources
are numbered. The `numbered` attribute does not override this setting.

In other words, the `numbered: false` can force a figure to not be numbered (if
figures are being numbered), but `numbered: true` cannot force a figure to be
numbered if figures are not being numbered.

There is no reason to ever explicitly set `numbered: true`, since this is the
default. The `numbered` attribute only exists so that it can be overridden to
`false`, to explicitly turn off figure numbering for a particular figure.

Finally, if `numbered` is `false` on a figure, then there are the following
consequences:

1. The figure will not show up in any list of figures, if there is one.
2. You cannot make certain kinds of smart crosslinks to it, since it has no
   number.

A resource without a number but with a title exists in a kind of limbo: it
is not a full-fledged figure which is listed in a list of figures and can be
easily referred to with smart crosslinks, but it's also not not-a-figure: it
is laid out like a figure, and can have a caption, etc.

## Resource alt text (M)

A resource can have alt text. The resource location does not matter: local, web
and inline resources all support alt text.

Alt text is text which is intended to take the place of the resource if the
resource itself cannot be seen. In the case of images, the obvious use case is
for readers with visual disabilities who are using a screen reader, but it also
includes audiobooks and ebook readers which often do not support embedded
images, audio and video, and which may have a hard time displaying math.

Here's an example of good alt text:

~~~
![a red apple, possibly a McIntosh or Spartan](fruit.jpg)
~~~

You can also use an attribute list:

~~~
{alt: "a red apple, possibly a McIntosh or Spartan"}
![](fruit.jpg)
~~~

The alt text should **not** have the same content as the resource figure title,
if the resource title is present. (Imagine the annoyance for someone with a
visual disability having their screen reader read identical alt text and figure
titles to them throughout an entire book!)

Instead, the alt text should be descriptive of the image content, while the
resource figure title can be more creative. For example, a figure title may be
"Washington Crossing the Delaware" and the alt text could be "Denzel Washington
on a boat in a river." Having good alt text would enable readers who cannot see
the image to still get the joke which the figure title makes.

Figure titles and alt text can, and should, be both added to resources. These
are some examples of resources with both alt text and a figure title:

~~~
![a red apple](mac.jpg "The Original Mac")

{alt: "a red apple", title: "The Original Mac"}
![](mac.jpg)

{alt: "a blue circle", title: "Earth From Space (Simplified)"}
```!
<svg width="20" height="20">
  <circle cx="10" cy="10" r="9" fill="blue"/>
</svg>
```
~~~




## Resource types (M)

The type of a resource can be specified by the `type` attribute, or inferred
from the format of the resource.

There are eight types of resources:

1. `audio`
2. `code`
3. `iframe`
4. `image`
5. `math`
6. `table`
7. `verbatim`
8. `video`

Each type of resource has a number of supported formats. Any of the resource
types can be inserted as a local resource or web resource, and many of the
resource types can also be inserted as an inline resource.

The `code`, `image` and `table` resource types are an extension and a
generalization of what is already supported in Markdown; the `audio`, `iframe`,
`math`, `verbatim` and `video` resource types are new.

Either way, each type of resource has a number of supported formats. Any of
the resource types can be inserted as a local resource or web resource,
and many of the resource types can also be inserted as an inline resource.

Resources are *not* parsed as plain [textual content](#textual-content);
instead, they are parsed according to the rules that govern the parsing of
resources.

All of these types of resources are discussed later.



## Resource formats (M)

The **Format** is the format of the resource. It can be specified with the
`type` attribute, or inferred from the file extension (discussed below).
This is important information to help the Markua Processor handle the
resource correctly.

Both the type and the format can be specified in an attribute list, by the
respective `type` and `format` attributes.

The type and the format can also be inferred from the file extension and, in the
case of web resources, the URL.

Markua Processors must interpret all unspecified file extensions as specifying a
resource of type `code` with a format of `guess`, unless the resource is a web
resource.

If the type and format are not specified and the resource is a web resource, the
Markua Processor may use the domain to decide what type of resource to assume.
For example, a domain of `youtube.com` may be assumed to be of type `video`, a
domain of `instagram.com` may be assumed to be of type `image`, and a domain of
`github.com` may be assumed to be of type `code`.

If the type is not specified in the attribute list, the format determines the
type. The formats can either be specified by the `format` attribute or (in most
cases) inferred from the file extension for local and web resources. (Inline
resources obviously have no file extension, since they are contained in the body
of a Markua document.)

As an author, all you typically do is provide the correct file extension for a
local resource or set the format in the attribute list. Markua recognizes the
format, and uses it to determine the type. If the format is unrecognized, then
the resource is treated as a resource of type `code` and with a format of
`guess`.

It's important to emphasize that the type and format of a resource can be
overridden using an attribute list. The file extensions just set the default
type and format that are inferred.

In rare instances, it is useful to override the `type` and `format` which have
been inferred by the Markua Processor based on the file extension of the
resource. This is done by specifying a type and/or format in the attribute list
of the resource.



## Resource classes (M)

As shown above, when a resource is inserted with a title, it is treated as a
figure. Figures can show up in one of the lists of figures which can be
generated near the start of the book or course.

If a resource has a `title` and thus is a figure, and if figures are listed
in groups (if the `list-figures` document setting is `groups`), the default is
to group by the `type` of the resource. However, the `class` attribute
can be used to override which list the resource appears in.




## Resource locations (M)

The **Location** is the location of the resource, when encountered by the
Markua Processor. There is no attribute for this; it is inferred from the
location.
* An inline resource is included directly in a Markua manuscript file.
* A local resource is located in the `resources` directory which accompanies
the Markua manuscript (or in a similar conceptual location for a web-based
Markua Processor).
* A web resource is on the web, accessed via `http://` or `https://`.

A resource is either considered a local, web or inline resource based on its
location:

Local Resource
: The resource is stored along with the manuscript--either in a `resources`
directory on a local filesystem, or uploaded to the same web service where the
Markua document is being written.

Web Resource
: The resource is referred to via an `http` or `https` URL.

Inline Resource
: The resource is defined right in the body of a Markua document.

### Local resources

If local resources are used, all local resources must be stored inside a
`resources` directory, or one of its subdirectories. The `resources` directory
is not part of the path to the resource.

Here's how the paths to local resources work:

1. An image called `foo.jpg` in the `resources` directory should be referred to
   as `![](foo.jpg)`, but can also be referred to as `![](resources/foo.jpg)`.
2. An image called `bar.png` in a subdirectory `images` of the `resources`
   directory should be referred to as `![](images/bar.png)`, but can also be
   referred to as `![](resources/images/foo.jpg)`.
3. For security reasons, leading slashes `/` and navigating upward (`../`) are
   not allowed: `![](/foo.jpg)`, `![](/images/bar.png)` and `![](../foo.jpg)`
   are all illegal.

The reason that paths can either include or omit the resources directory is
simple: including it makes it a simple relative path, which means that
Markdown-aware tools that support, say, external images will just work. However,
omitting it is nice to type, so this is something which should be supported as
well. And the reason the resources directory exists is to keep the Markua
manuscript file(s) separate from the resources, to reduce clutter.

Nested directory trees work as well. A file called `foo.rb` in a
`ch1/examples/ruby` directory tree inside the `resources` directory is referenced
as `![](ch1/examples/ruby/foo.rb)` or as `![](resources/ch1/examples/ruby/foo.rb)`.

Markua does not specify whether there are any subdirectories of the `resources`
directory, or what their names are. Since any subdirectories have their names as
part of the path to the resource, authors can do whatever they want. For example,
you can create subdirectories of the `resources` directory for different types of
resource, such as `audio`, `code`, `images`, etc., but you can also just put them
all in the `resources` directory together. To be clear: the names of the
directories have no meaning, and do not restrict the formats of what can go inside
them.

If you are using a hosted service to write in Markua, this service can store
resources wherever it wants. However, if they provide a download (say as a zip
file) they should create the resources directory and provide the uploaded
resources in that directory. If a nested structure is used, it should be exported
that way--if a web service produces paths which reference images inside an images
directory (e.g. as `images/foo.png`), then the zip file containing an export
should contain a `resources` directory which contains an `images` subdirectory
with the images.

### Web resources

If web resources are supported, both `http:` and `https:` resources should be
supported.

Web resources are identified by the absolute URL of the resource on the internet.

### Inline resources

Certain types of resources can be inserted inline in a Markua document:

* `code`
* `image` (of SVG format only, discussed later)
* `list`
* `math` (of `latexmath` or `asciimath` format, discussed later)
* `table`
* `verbatim`

</div>



# Blocks and inlines

We can think of a document as a sequence of
[blocks](@)---structural elements like paragraphs, block
quotations, lists, headings, rules, and code blocks.  Some blocks (like
block quotes and list items) contain other blocks; others (like
headings and paragraphs) contain [inline](@) content---text,
links, emphasized text, images, code spans, and so on.

## Precedence

Indicators of block structure always take precedence over indicators
of inline structure.  So, for example, the following is a list with
two items, not a list with one item containing a code span:

```````````````````````````````` example
- `one
- two`
.
<ul>
<li>`one</li>
<li>two`</li>
</ul>
````````````````````````````````


This means that parsing can proceed in two steps:  first, the block
structure of the document can be discerned; second, text lines inside
paragraphs, headings, and other block constructs can be parsed for inline
structure.  The second step requires information about link reference
definitions that will be available only at the end of the first
step.  Note that the first step requires processing lines in sequence,
but the second can be parallelized, since the inline parsing of
one block element does not affect the inline parsing of any other.

## Container blocks and leaf blocks

We can divide blocks into two types:
[container blocks](#container-blocks),
which can contain other blocks, and [leaf blocks](#leaf-blocks),
which cannot.



# Leaf blocks

This section describes the different kinds of leaf block that make up a
Markdown document.

## Thematic breaks

A line consisting of optionally up to three spaces of indentation, followed by a
sequence of three or more matching `-`, `_`, or `*` characters, each followed
optionally by any number of spaces or tabs, forms a
[thematic break](@).

```````````````````````````````` example
***
---
___
.
<hr />
<hr />
<hr />
````````````````````````````````


Wrong characters:

```````````````````````````````` example
+++
.
<p>+++</p>
````````````````````````````````


```````````````````````````````` example
===
.
<p>===</p>
````````````````````````````````


Not enough characters:

```````````````````````````````` example
--
**
__
.
<p>--
**
__</p>
````````````````````````````````


Up to three spaces of indentation are allowed:

```````````````````````````````` example
 ***
  ***
   ***
.
<hr />
<hr />
<hr />
````````````````````````````````


Four spaces of indentation is too many:

```````````````````````````````` example
    ***
.
<pre><code>***
</code></pre>
````````````````````````````````


```````````````````````````````` example
Foo
    ***
.
<p>Foo
***</p>
````````````````````````````````


More than three characters may be used:

```````````````````````````````` example
_____________________________________
.
<hr />
````````````````````````````````


Spaces and tabs are allowed between the characters:

```````````````````````````````` example
 - - -
.
<hr />
````````````````````````````````


```````````````````````````````` example
 **  * ** * ** * **
.
<hr />
````````````````````````````````


```````````````````````````````` example
-     -      -      -
.
<hr />
````````````````````````````````


Spaces and tabs are allowed at the end:

```````````````````````````````` example
- - - -    
.
<hr />
````````````````````````````````


However, no other characters may occur in the line:

```````````````````````````````` example
_ _ _ _ a

a------

---a---
.
<p>_ _ _ _ a</p>
<p>a------</p>
<p>---a---</p>
````````````````````````````````


It is required that all of the characters other than spaces or tabs be the same.
So, this is not a thematic break:

```````````````````````````````` example
 *-*
.
<p><em>-</em></p>
````````````````````````````````


Thematic breaks do not need blank lines before or after:

```````````````````````````````` example
- foo
***
- bar
.
<ul>
<li>foo</li>
</ul>
<hr />
<ul>
<li>bar</li>
</ul>
````````````````````````````````


Thematic breaks can interrupt a paragraph:

```````````````````````````````` example
Foo
***
bar
.
<p>Foo</p>
<hr />
<p>bar</p>
````````````````````````````````


If a line of dashes that meets the above conditions for being a
thematic break could also be interpreted as the underline of a [setext
heading], the interpretation as a
[setext heading] takes precedence. Thus, for example,
this is a setext heading, not a paragraph followed by a thematic break:

```````````````````````````````` example
Foo
---
bar
.
<h2>Foo</h2>
<p>bar</p>
````````````````````````````````


When both a thematic break and a list item are possible
interpretations of a line, the thematic break takes precedence:

```````````````````````````````` example
* Foo
* * *
* Bar
.
<ul>
<li>Foo</li>
</ul>
<hr />
<ul>
<li>Bar</li>
</ul>
````````````````````````````````


If you want a thematic break in a list item, use a different bullet:

```````````````````````````````` example
- Foo
- * * *
.
<ul>
<li>Foo</li>
<li>
<hr />
</li>
</ul>
````````````````````````````````


## ATX headings

An [ATX heading](@)
consists of a string of characters, parsed as inline content, between an
opening sequence of 1--6 unescaped `#` characters and an optional
closing sequence of any number of unescaped `#` characters.
The opening sequence of `#` characters must be followed by spaces or tabs, or
by the end of line. The optional closing sequence of `#`s must be preceded by
spaces or tabs and may be followed by spaces or tabs only.  The opening
`#` character may be preceded by up to three spaces of indentation.  The raw
contents of the heading are stripped of leading and trailing space or tabs
before being parsed as inline content.  The heading level is equal to the number
of `#` characters in the opening sequence.

Simple headings:

```````````````````````````````` example
# foo
## foo
### foo
#### foo
##### foo
###### foo
.
<h1>foo</h1>
<h2>foo</h2>
<h3>foo</h3>
<h4>foo</h4>
<h5>foo</h5>
<h6>foo</h6>
````````````````````````````````


More than six `#` characters is not a heading:

```````````````````````````````` example
####### foo
.
<p>####### foo</p>
````````````````````````````````


At least one space or tab is required between the `#` characters and the
heading's contents, unless the heading is empty.  Note that many
implementations currently do not require the space.  However, the
space was required by the
[original ATX implementation](http://www.aaronsw.com/2002/atx/atx.py),
and it helps prevent things like the following from being parsed as
headings:

```````````````````````````````` example
#5 bolt

#hashtag
.
<p>#5 bolt</p>
<p>#hashtag</p>
````````````````````````````````


This is not a heading, because the first `#` is escaped:

```````````````````````````````` example
\## foo
.
<p>## foo</p>
````````````````````````````````


Contents are parsed as inlines:

```````````````````````````````` example
# foo *bar* \*baz\*
.
<h1>foo <em>bar</em> *baz*</h1>
````````````````````````````````


Leading and trailing spaces or tabs are ignored in parsing inline content:

```````````````````````````````` example
#                  foo                     
.
<h1>foo</h1>
````````````````````````````````


Up to three spaces of indentation are allowed:

```````````````````````````````` example
 ### foo
  ## foo
   # foo
.
<h3>foo</h3>
<h2>foo</h2>
<h1>foo</h1>
````````````````````````````````


Four spaces of indentation is too many:

```````````````````````````````` example
    # foo
.
<pre><code># foo
</code></pre>
````````````````````````````````


```````````````````````````````` example
foo
    # bar
.
<p>foo
# bar</p>
````````````````````````````````


A closing sequence of `#` characters is optional:

```````````````````````````````` example
## foo ##
  ###   bar    ###
.
<h2>foo</h2>
<h3>bar</h3>
````````````````````````````````


It need not be the same length as the opening sequence:

```````````````````````````````` example
# foo ##################################
##### foo ##
.
<h1>foo</h1>
<h5>foo</h5>
````````````````````````````````


Spaces or tabs are allowed after the closing sequence:

```````````````````````````````` example
### foo ###     
.
<h3>foo</h3>
````````````````````````````````


A sequence of `#` characters with anything but spaces or tabs following it
is not a closing sequence, but counts as part of the contents of the
heading:

```````````````````````````````` example
### foo ### b
.
<h3>foo ### b</h3>
````````````````````````````````


The closing sequence must be preceded by a space or tab:

```````````````````````````````` example
# foo#
.
<h1>foo#</h1>
````````````````````````````````


Backslash-escaped `#` characters do not count as part
of the closing sequence:

```````````````````````````````` example
### foo \###
## foo #\##
# foo \#
.
<h3>foo ###</h3>
<h2>foo ###</h2>
<h1>foo #</h1>
````````````````````````````````


ATX headings need not be separated from surrounding content by blank
lines, and they can interrupt paragraphs:

```````````````````````````````` example
****
## foo
****
.
<hr />
<h2>foo</h2>
<hr />
````````````````````````````````


```````````````````````````````` example
Foo bar
# baz
Bar foo
.
<p>Foo bar</p>
<h1>baz</h1>
<p>Bar foo</p>
````````````````````````````````


ATX headings can be empty:

```````````````````````````````` example
## 
#
### ###
.
<h2></h2>
<h1></h1>
<h3></h3>
````````````````````````````````


## Setext headings

A [setext heading](@) consists of one or more
lines of text, not interrupted by a blank line, of which the first line does not
have more than 3 spaces of indentation, followed by
a [setext heading underline].  The lines of text must be such
that, were they not followed by the setext heading underline,
they would be interpreted as a paragraph:  they cannot be
interpretable as a [code fence], [ATX heading][ATX headings],
[block quote][block quotes], [thematic break][thematic breaks],
[list item][list items], or [HTML block][HTML blocks].

A [setext heading underline](@) is a sequence of
`=` characters or a sequence of `-` characters, with no more than 3
spaces of indentation and any number of trailing spaces or tabs.

The heading is a level 1 heading if `=` characters are used in
the [setext heading underline], and a level 2 heading if `-`
characters are used.  The contents of the heading are the result
of parsing the preceding lines of text as CommonMark inline
content.

In general, a setext heading need not be preceded or followed by a
blank line.  However, it cannot interrupt a paragraph, so when a
setext heading comes after a paragraph, a blank line is needed between
them.

Simple examples:

```````````````````````````````` example
Foo *bar*
=========

Foo *bar*
---------
.
<h1>Foo <em>bar</em></h1>
<h2>Foo <em>bar</em></h2>
````````````````````````````````


The content of the header may span more than one line:

```````````````````````````````` example
Foo *bar
baz*
====
.
<h1>Foo <em>bar
baz</em></h1>
````````````````````````````````

The contents are the result of parsing the headings's raw
content as inlines.  The heading's raw content is formed by
concatenating the lines and removing initial and final
spaces or tabs.

```````````````````````````````` example
  Foo *bar
baz*→
====
.
<h1>Foo <em>bar
baz</em></h1>
````````````````````````````````


The underlining can be any length:

```````````````````````````````` example
Foo
-------------------------

Foo
=
.
<h2>Foo</h2>
<h1>Foo</h1>
````````````````````````````````


The heading content can be preceded by up to three spaces of indentation, and
need not line up with the underlining:

```````````````````````````````` example
   Foo
---

  Foo
-----

  Foo
  ===
.
<h2>Foo</h2>
<h2>Foo</h2>
<h1>Foo</h1>
````````````````````````````````


Four spaces of indentation is too many:

```````````````````````````````` example
    Foo
    ---

    Foo
---
.
<pre><code>Foo
---

Foo
</code></pre>
<hr />
````````````````````````````````


The setext heading underline can be preceded by up to three spaces of
indentation, and may have trailing spaces or tabs:

```````````````````````````````` example
Foo
   ----      
.
<h2>Foo</h2>
````````````````````````````````


Four spaces of indentation is too many:

```````````````````````````````` example
Foo
    ---
.
<p>Foo
---</p>
````````````````````````````````


The setext heading underline cannot contain internal spaces or tabs:

```````````````````````````````` example
Foo
= =

Foo
--- -
.
<p>Foo
= =</p>
<p>Foo</p>
<hr />
````````````````````````````````


Trailing spaces or tabs in the content line do not cause a hard line break:

```````````````````````````````` example
Foo  
-----
.
<h2>Foo</h2>
````````````````````````````````


Nor does a backslash at the end:

```````````````````````````````` example
Foo\
----
.
<h2>Foo\</h2>
````````````````````````````````


Since indicators of block structure take precedence over
indicators of inline structure, the following are setext headings:

```````````````````````````````` example
`Foo
----
`

<a title="a lot
---
of dashes"/>
.
<h2>`Foo</h2>
<p>`</p>
<h2>&lt;a title=&quot;a lot</h2>
<p>of dashes&quot;/&gt;</p>
````````````````````````````````


The setext heading underline cannot be a [lazy continuation
line] in a list item or block quote:

```````````````````````````````` example
> Foo
---
.
<blockquote>
<p>Foo</p>
</blockquote>
<hr />
````````````````````````````````


```````````````````````````````` example
> foo
bar
===
.
<blockquote>
<p>foo
bar
===</p>
</blockquote>
````````````````````````````````


```````````````````````````````` example
- Foo
---
.
<ul>
<li>Foo</li>
</ul>
<hr />
````````````````````````````````


A blank line is needed between a paragraph and a following
setext heading, since otherwise the paragraph becomes part
of the heading's content:

```````````````````````````````` example
Foo
Bar
---
.
<h2>Foo
Bar</h2>
````````````````````````````````


But in general a blank line is not required before or after
setext headings:

```````````````````````````````` example
---
Foo
---
Bar
---
Baz
.
<hr />
<h2>Foo</h2>
<h2>Bar</h2>
<p>Baz</p>
````````````````````````````````


Setext headings cannot be empty:

```````````````````````````````` example

====
.
<p>====</p>
````````````````````````````````


Setext heading text lines must not be interpretable as block
constructs other than paragraphs.  So, the line of dashes
in these examples gets interpreted as a thematic break:

```````````````````````````````` example
---
---
.
<hr />
<hr />
````````````````````````````````


```````````````````````````````` example
- foo
-----
.
<ul>
<li>foo</li>
</ul>
<hr />
````````````````````````````````


```````````````````````````````` example
    foo
---
.
<pre><code>foo
</code></pre>
<hr />
````````````````````````````````


```````````````````````````````` example
> foo
-----
.
<blockquote>
<p>foo</p>
</blockquote>
<hr />
````````````````````````````````


If you want a heading with `> foo` as its literal text, you can
use backslash escapes:

```````````````````````````````` example
\> foo
------
.
<h2>&gt; foo</h2>
````````````````````````````````


**Compatibility note:**  Most existing Markdown implementations
do not allow the text of setext headings to span multiple lines.
But there is no consensus about how to interpret

``` markdown
Foo
bar
---
baz
```

One can find four different interpretations:

1. paragraph "Foo", heading "bar", paragraph "baz"
2. paragraph "Foo bar", thematic break, paragraph "baz"
3. paragraph "Foo bar --- baz"
4. heading "Foo bar", paragraph "baz"

We find interpretation 4 most natural, and interpretation 4
increases the expressive power of CommonMark, by allowing
multiline headings.  Authors who want interpretation 1 can
put a blank line after the first paragraph:

```````````````````````````````` example
Foo

bar
---
baz
.
<p>Foo</p>
<h2>bar</h2>
<p>baz</p>
````````````````````````````````


Authors who want interpretation 2 can put blank lines around
the thematic break,

```````````````````````````````` example
Foo
bar

---

baz
.
<p>Foo
bar</p>
<hr />
<p>baz</p>
````````````````````````````````


or use a thematic break that cannot count as a [setext heading
underline], such as

```````````````````````````````` example
Foo
bar
* * *
baz
.
<p>Foo
bar</p>
<hr />
<p>baz</p>
````````````````````````````````


Authors who want interpretation 3 can use backslash escapes:

```````````````````````````````` example
Foo
bar
\---
baz
.
<p>Foo
bar
---
baz</p>
````````````````````````````````


## Indented code blocks

An [indented code block](@) is composed of one or more
[indented chunks] separated by blank lines.
An [indented chunk](@) is a sequence of non-blank lines,
each preceded by four or more spaces of indentation. The contents of the code
block are the literal contents of the lines, including trailing
[line endings], minus four spaces of indentation.
An indented code block has no [info string].

An indented code block cannot interrupt a paragraph, so there must be
a blank line between a paragraph and a following indented code block.
(A blank line is not needed, however, between a code block and a following
paragraph.)

```````````````````````````````` example
    a simple
      indented code block
.
<pre><code>a simple
  indented code block
</code></pre>
````````````````````````````````


If there is any ambiguity between an interpretation of indentation
as a code block and as indicating that material belongs to a [list
item][list items], the list item interpretation takes precedence:

```````````````````````````````` example
  - foo

    bar
.
<ul>
<li>
<p>foo</p>
<p>bar</p>
</li>
</ul>
````````````````````````````````


```````````````````````````````` example
1.  foo

    - bar
.
<ol>
<li>
<p>foo</p>
<ul>
<li>bar</li>
</ul>
</li>
</ol>
````````````````````````````````



The contents of a code block are literal text, and do not get parsed
as Markdown:

```````````````````````````````` example
    <a/>
    *hi*

    - one
.
<pre><code>&lt;a/&gt;
*hi*

- one
</code></pre>
````````````````````````````````


Here we have three chunks separated by blank lines:

```````````````````````````````` example
    chunk1

    chunk2
  
 
 
    chunk3
.
<pre><code>chunk1

chunk2



chunk3
</code></pre>
````````````````````````````````


Any initial spaces or tabs beyond four spaces of indentation will be included in
the content, even in interior blank lines:

```````````````````````````````` example
    chunk1
      
      chunk2
.
<pre><code>chunk1
  
  chunk2
</code></pre>
````````````````````````````````


An indented code block cannot interrupt a paragraph.  (This
allows hanging indents and the like.)

```````````````````````````````` example
Foo
    bar

.
<p>Foo
bar</p>
````````````````````````````````


However, any non-blank line with fewer than four spaces of indentation ends
the code block immediately.  So a paragraph may occur immediately
after indented code:

```````````````````````````````` example
    foo
bar
.
<pre><code>foo
</code></pre>
<p>bar</p>
````````````````````````````````


And indented code can occur immediately before and after other kinds of
blocks:

```````````````````````````````` example
# Heading
    foo
Heading
------
    foo
----
.
<h1>Heading</h1>
<pre><code>foo
</code></pre>
<h2>Heading</h2>
<pre><code>foo
</code></pre>
<hr />
````````````````````````````````


The first line can be preceded by more than four spaces of indentation:

```````````````````````````````` example
        foo
    bar
.
<pre><code>    foo
bar
</code></pre>
````````````````````````````````


Blank lines preceding or following an indented code block
are not included in it:

```````````````````````````````` example

    
    foo
    

.
<pre><code>foo
</code></pre>
````````````````````````````````


Trailing spaces or tabs are included in the code block's content:

```````````````````````````````` example
    foo  
.
<pre><code>foo  
</code></pre>
````````````````````````````````



## Fenced code blocks

A [code fence](@) is a sequence
of at least three consecutive backtick characters (`` ` ``) or
tildes (`~`).  (Tildes and backticks cannot be mixed.)
A [fenced code block](@)
begins with a code fence, preceded by up to three spaces of indentation.

The line with the opening code fence may optionally contain some text
following the code fence; this is trimmed of leading and trailing
spaces or tabs and called the [info string](@). If the [info string] comes
after a backtick fence, it may not contain any backtick
characters.  (The reason for this restriction is that otherwise
some inline code would be incorrectly interpreted as the
beginning of a fenced code block.)

The content of the code block consists of all subsequent lines, until
a closing [code fence] of the same type as the code block
began with (backticks or tildes), and with at least as many backticks
or tildes as the opening code fence.  If the leading code fence is
preceded by N spaces of indentation, then up to N spaces of indentation are
removed from each line of the content (if present).  (If a content line is not
indented, it is preserved unchanged.  If it is indented N spaces or less, all
of the indentation is removed.)

The closing code fence may be preceded by up to three spaces of indentation, and
may be followed only by spaces or tabs, which are ignored.  If the end of the
containing block (or document) is reached and no closing code fence
has been found, the code block contains all of the lines after the
opening code fence until the end of the containing block (or
document).  (An alternative spec would require backtracking in the
event that a closing code fence is not found.  But this makes parsing
much less efficient, and there seems to be no real down side to the
behavior described here.)

A fenced code block may interrupt a paragraph, and does not require
a blank line either before or after.

The content of a code fence is treated as literal text, not parsed
as inlines.  The first word of the [info string] is typically used to
specify the language of the code sample, and rendered in the `class`
attribute of the `code` tag.  However, this spec does not mandate any
particular treatment of the [info string].

Here is a simple example with backticks:

```````````````````````````````` example
```
<
 >
```
.
<pre><code>&lt;
 &gt;
</code></pre>
````````````````````````````````


With tildes:

```````````````````````````````` example
~~~
<
 >
~~~
.
<pre><code>&lt;
 &gt;
</code></pre>
````````````````````````````````

Fewer than three backticks is not enough:

```````````````````````````````` example
``
foo
``
.
<p><code>foo</code></p>
````````````````````````````````

The closing code fence must use the same character as the opening
fence:

```````````````````````````````` example
```
aaa
~~~
```
.
<pre><code>aaa
~~~
</code></pre>
````````````````````````````````


```````````````````````````````` example
~~~
aaa
```
~~~
.
<pre><code>aaa
```
</code></pre>
````````````````````````````````


The closing code fence must be at least as long as the opening fence:

```````````````````````````````` example
````
aaa
```
``````
.
<pre><code>aaa
```
</code></pre>
````````````````````````````````


```````````````````````````````` example
~~~~
aaa
~~~
~~~~
.
<pre><code>aaa
~~~
</code></pre>
````````````````````````````````


Unclosed code blocks are closed by the end of the document
(or the enclosing [block quote][block quotes] or [list item][list items]):

```````````````````````````````` example
```
.
<pre><code></code></pre>
````````````````````````````````


```````````````````````````````` example
`````

```
aaa
.
<pre><code>
```
aaa
</code></pre>
````````````````````````````````


```````````````````````````````` example
> ```
> aaa

bbb
.
<blockquote>
<pre><code>aaa
</code></pre>
</blockquote>
<p>bbb</p>
````````````````````````````````


A code block can have all empty lines as its content:

```````````````````````````````` example
```

  
```
.
<pre><code>
  
</code></pre>
````````````````````````````````


A code block can be empty:

```````````````````````````````` example
```
```
.
<pre><code></code></pre>
````````````````````````````````


Fences can be indented.  If the opening fence is indented,
content lines will have equivalent opening indentation removed,
if present:

```````````````````````````````` example
 ```
 aaa
aaa
```
.
<pre><code>aaa
aaa
</code></pre>
````````````````````````````````


```````````````````````````````` example
  ```
aaa
  aaa
aaa
  ```
.
<pre><code>aaa
aaa
aaa
</code></pre>
````````````````````````````````


```````````````````````````````` example
   ```
   aaa
    aaa
  aaa
   ```
.
<pre><code>aaa
 aaa
aaa
</code></pre>
````````````````````````````````


Four spaces of indentation is too many:

```````````````````````````````` example
    ```
    aaa
    ```
.
<pre><code>```
aaa
```
</code></pre>
````````````````````````````````


Closing fences may be preceded by up to three spaces of indentation, and their
indentation need not match that of the opening fence:

```````````````````````````````` example
```
aaa
  ```
.
<pre><code>aaa
</code></pre>
````````````````````````````````


```````````````````````````````` example
   ```
aaa
  ```
.
<pre><code>aaa
</code></pre>
````````````````````````````````


This is not a closing fence, because it is indented 4 spaces:

```````````````````````````````` example
```
aaa
    ```
.
<pre><code>aaa
    ```
</code></pre>
````````````````````````````````



Code fences (opening and closing) cannot contain internal spaces or tabs:

```````````````````````````````` example
``` ```
aaa
.
<p><code> </code>
aaa</p>
````````````````````````````````


```````````````````````````````` example
~~~~~~
aaa
~~~ ~~
.
<pre><code>aaa
~~~ ~~
</code></pre>
````````````````````````````````


Fenced code blocks can interrupt paragraphs, and can be followed
directly by paragraphs, without a blank line between:

```````````````````````````````` example
foo
```
bar
```
baz
.
<p>foo</p>
<pre><code>bar
</code></pre>
<p>baz</p>
````````````````````````````````


Other blocks can also occur before and after fenced code blocks
without an intervening blank line:

```````````````````````````````` example
foo
---
~~~
bar
~~~
# baz
.
<h2>foo</h2>
<pre><code>bar
</code></pre>
<h1>baz</h1>
````````````````````````````````


An [info string] can be provided after the opening code fence.
Although this spec doesn't mandate any particular treatment of
the info string, the first word is typically used to specify
the language of the code block. In HTML output, the language is
normally indicated by adding a class to the `code` element consisting
of `language-` followed by the language name.

```````````````````````````````` example
```ruby
def foo(x)
  return 3
end
```
.
<pre><code class="language-ruby">def foo(x)
  return 3
end
</code></pre>
````````````````````````````````


```````````````````````````````` example
~~~~    ruby startline=3 $%@#$
def foo(x)
  return 3
end
~~~~~~~
.
<pre><code class="language-ruby">def foo(x)
  return 3
end
</code></pre>
````````````````````````````````


```````````````````````````````` example
````;
````
.
<pre><code class="language-;"></code></pre>
````````````````````````````````


[Info strings] for backtick code blocks cannot contain backticks:

```````````````````````````````` example
``` aa ```
foo
.
<p><code>aa</code>
foo</p>
````````````````````````````````


[Info strings] for tilde code blocks can contain backticks and tildes:

```````````````````````````````` example
~~~ aa ``` ~~~
foo
~~~
.
<pre><code class="language-aa">foo
</code></pre>
````````````````````````````````


Closing code fences cannot have [info strings]:

```````````````````````````````` example
```
``` aaa
```
.
<pre><code>``` aaa
</code></pre>
````````````````````````````````



<div class="extension">

## Code resources (M)

Code can be a local, web or inline resource, just like any other resource, and
the same resource syntax applies to code as to all other resources.

Code cannot have alt text. It's just text. If any alt text is provided for a
code resource, it is ignored.

Markua specifies only one specific file extension to be associated with a type
of `code`: the `.txt` extension, which is for the format of `text`. However,
Markua Processors must interpret **all** unspecified file extensions as
specifying a resource of type `code` with a format of `guess`.

This way, the decision about the language recognized does not need to be done
by the Markua Processor, but can instead be delegated to the syntax highlighter,
such as Pygments.

Regardless of whether syntax highlighting is supported and the programming
language is detected, all code must be formatted as `monospaced text` by Markua
Processors.

The `text` format means to not do any syntax highlighting as well.

The `guess` format is a request for the Markua Processor to guess at the
programming language based on the file extension and/or the syntax of the code
itself. Then, if the detected language corresponds to a particular programming
language which the Markua Processor recognizes, and if the Markua Processor
supports syntax highlighting, then it can format the resource as nicely
syntax-highlighted code. Syntax highlighting is entirely optional in Markua
Processors. If a Markua Processor does not support syntax highlighting, and/or
if it cannot detect a matching supported programming language, then it must
format the code as though the format was `text`--i.e. to format it as
unformatted monospaced text.

Besides the `text` and `guess` values of the format attribute, you can also
specify the programming language by setting the format attribute to a specific
programming language. This is more reliable than `guess`. Unlike other resource
types, Markua does not specify the complete set of the values of the `format`
attribute--there are so many programming languages in the world, and new ones
are added so frequently, that doing so would be impractical.

However, while a complete set of the values of the `format` attribute is not
specified, Markua does specify the `console` value of the `format` attribute to
indicate console input. A Markua Processor should format console input as such.
(For example, Leanpub uses the open source Pygments library for its code
formatting, and Pygments handles `console` format correctly, so Leanpub gets this
for free.)

The default value of the format attribute for code is complex:

1. For code which is inserted as a span (which is only supported with inline
   resources), the default format is `text`.
2. For code which is inserted as a figure which is inserted as an inline
   resource using three tildes, the default format is `text`.
3. For all other code, the default format is `guess`. This includes local and web
   resources inserted as figures, and code inserted as an inline figure using
   three backticks.

Note that the default format can be overridden by specifying it via an attribute
list, or after the three backticks in syntactic sugar.

### Supported Attributes for Code

The following are the supported attributes for code resources, in addition to
the `class`, `format`, `title` and `type` attributes which all resources
support.

`line-numbers`
: This determines whether the code sample shows line numbers. Legal values are
`true` or `false`. The default value is `false`. Any value other than `true` is
interpreted as `false`.

`number-from`
: If line numbers are shown, this lets you override the starting number of the
line numbers. The default value is `1`.

`crop-start`
: Sometimes it's desirable to only show part of a code resource defined in an
external file as the code example. The `crop-start` and `crop-end` attributes let
you accomplish this. The `crop-start` attribute defines the line which represents
the first line included from the resource. For example, `{crop-start: 10,
crop-end: 15, line-numbers: true, number-from: 10}` ensures that lines 10-15 are
shown and are numbered as lines 10-15. The default value is 1, which is the first
line of the file.

`crop-end`
: This attribute ends the range started with crop-start. The default value of
`crop-end` is to be omitted, which is equivalent to specifying the last line of
the file.

#### Default Value of the `format` attribute in Inline Code Samples

The default value of the `format` attribute for a code resource inserted as a
figure varies based on context.

If the code resource is a local or web resource, it defaults to `guess`.

If the code resource is an inline resource, the default varies based on the
delimiter, and whether the code is inserted as a span or as a block.

With three backticks the default format is `guess`, and with three tildes, the
default format is `text`. This way, you can vary the default without having to
type an attribute list: if you want the code language guessed at, use backticks;
if you don't, use tildes. Of course, you can specify any attributes you wish
with either delimiter, and specified attributes override default ones. The only
reason there are different defaults are to make things easier to type.
Programmers refer to such niceties as "syntactic sugar".

The default value of block code resources inserted with three backticks can be
overridden from `guess` to some other value by setting by the
`default-code-language` attribute on the entire Markua document. (This attribute
has no effect on resources inserted with three tildes, or on code spans.)

### Local Code Resources

Local code resources can be inserted as a figure.

This first figure will be a type of code and a format of `guess`. A Markua
Processor which associates `.rb` file extensions with Ruby code will treat this
as Ruby code; a Markua Processor which has no association for `.rb` files will
treat it as plain text:

~~~
Here's a paragraph before the figure.

![](hello.rb "Hello World in Ruby")

Here's a paragraph after the figure.
~~~

That is equivalent to:

~~~
Here's a paragraph before the figure.

{format: guess}
![](hello.rb "Hello World in Ruby")

Here's a paragraph after the figure.
~~~

If you don't want to take chances you can do this:

~~~
Here's a paragraph before the figure.

{format: ruby}
![](hello.rb "Hello World in Ruby")

Here's a paragraph after the figure.
~~~

Note that the title is optional in all figures:

~~~
Here's a paragraph before the figure.

![](hello.rb)

Here's a paragraph after the figure.
~~~

### Web Code Resources

Web code resources function identically to how local code resources work,
including the significance of file extensions. The only differences is that the
files are on the web.

This will be a type of code and a format of `guess` since the file extension is
not specified:

~~~
![](https://markua.com/hello.rb "Hello World in Ruby")
~~~

That is equivalent to:

~~~
{format: guess}
![](https://markua.com/hello.rb "Hello World in Ruby")
~~~

If you don't want to take chances you can do this:

~~~
{format: ruby}
![](https://markua.com/hello.rb "Hello World in Ruby")
~~~

### Inline Code Resources

Inline code resources are the most flexible way to insert code. They are the only
way to insert code as a span resource, and the most straightforward way to add
short code examples as figures.

The great thing about inline code resources, either as spans or figures, is that
they work the same way as they do in CommonMark and GFM, with small additions
by Markua.

#### No Attribute Lists or Format Specifiers on Indented Code Blocks

Indented code blocks are supported for compatibility with CommonMark and GFM.
However, no attribute lists or format specifiers can be used. If you want to use
them, use a fenced code block.

#### Attribute Lists and Format Specifiers on Fenced Code Blocks

Fenced code blocks, discussed earlier, are how to insert inline code resources
as figures. These can have attribute lists or format specifiers.

This will be a type of code and a format of `guess` since three backticks are
used and since the format is not specified:

~~~
Some paragraph.

```
puts "hello"
```

Some paragraph.
~~~

That is equivalent to:

~~~
Some paragraph.

```guess
puts "hello"
```

Some paragraph.
~~~

If you don't want to take chances you can do this to explicitly specify the
format:

~~~
Some paragraph.

```ruby
puts "hello"
```

Some paragraph.
~~~

This Ruby code may be formatted as such if the Markua Processor understands
`ruby`. If not, the `ruby` format will be ignored.

If you don't like syntactic sugar you can do:

~~~
Some paragraph.

{format: ruby}
```
puts "hello"
```

Some paragraph.
~~~

If you want a figure title, you can add it to the attribute list with any of
the above. For example:

~~~
Some paragraph.

{title: "Hello World in Ruby"}
```ruby
puts "hello"
```

Some paragraph.
~~~

Finally, if you want the code to definitely not get syntax highlighted, you can
force format to be `text` in one of two ways.

First, you can set it explicitly:

~~~
Some paragraph.

```text
puts "hello"
```

Some paragraph.
~~~

Second, you can use three tildes instead of three backticks, since the default
with tildes is `text` not `guess`:

```
Some paragraph.

~~~
puts "hello"
~~~

Some paragraph.
```

As discussed previously, console input and output should be formatted as such by
a Markua Processor:

~~~
```console
$ git init
Initialized empty Git repository in /path/to/repo
```
~~~

Finally, it's important to note that when you are writing about other inline
formats, such as SVG or AsciiMath, what you are really doing is creating a code
resource. This is shown in the sections below, which discuss SVG and AsciiMath,
but this applies more broadly.

#### No Attribute Lists on Code Spans

Code spans (like `puts foo`) are discussed later in the spec. Code spans work
just as in CommonMark and GFM. Since Markua adds attribute lists to code
blocks, you may wonder whether Markua adds attribute lists to code spans.

**So, to be clear: Markua does NOT support attribute lists on code spans.**

So, in an example like the following, the attribute list is simply ignored:

~~~
Hello World in Ruby is a simple `puts "hello world"`{format: ruby} statement.
~~~

The reason for this is simple: it does not make sense to run a syntax
highlighter to format two or three words of code. If you want syntax
highlighting, use a code block.

### Marking Code as Added or Deleted

Markua supports marking code as added or deleted, which can be helpful if you are
writing a computer programming book and want to indicate what code should be
added or removed to a larger program.

The way to do this is to add special comment lines to your code.

The magic words are `markua-start-insert`, `markua-end-insert`,
`markua-start-delete` and `markua-end-delete`. Any line containing one of those
words will be removed completely by a Markua Processor before being inserted into
the output.

The Markua Processor will then be able to determine which code is being deleted
or inserted, and format it accordingly. The recommended way for a Markua
Processor to do this is to make code which is being inserted get **`bolded`**,
and to make code which is getting deleted to be put in ~~`strikethrough`~~.

Finally, while syntax highlighting is optional in a Markua Processor, if a Markua
Processor does support syntax highlighting it is allowed for the Markua Processor
to not do any syntax highlighting when there is the presence of any of any
special `markua-*` comments. Syntax highlighting may make it harder to notice the
added and removed code, if they are formatted with bold and strikethrough
respectively.

### Line Wrapping in Code Resources

Code resources should have newlines added by the author to ensure that automatic
line wrapping is not relied upon. Markua Processors may wrap lines to ensure that
all code is visible on a page, and *may* add continuation characters (like the
backslash `\` character) in the output to indicate that a line has been
automatically wrapped. However, adding a continuation character is not a
requirement, nor is the choice of which continuation character is used.

</div>



<div class="extension">

## Verbatim resources (M)

The `verbatim` resource type is used to preserve leading and internal whitespace
while still supporting Markua text formatting (things like `**bold**` and
`*italic*`).

**While Markua text formatting IS supported inside a verbatim resource, Markua
resources (including images) ARE NOT supported inside a verbatim resource.
While this may seem arbitrary and overly restrictive, we need to prevent a
slippery slope of expanded scope.**

The following are the verbatim resource formats and the file extensions which
choose them by default:

`verbatim`
: verbatim text - `.text`

The primary use case is poetry, and the secondary use case is boilerplate legal
text. If you don't care about leading or internal whitespace, or about poetry or
legalese, you can safely skip this section.

For a verbatim resource, leading whitespace, internal whitespace, and newlines
must all be preserved verbatim as typed. This can be useful for certain types of
poetry, such as indenting the last two lines of a sonnet.

To be clear:

* Trailing whitespace before a newline does not need to be preserved.
* This includes all leading and internal spaces and newlines regardless of the
number of consecutive leading, internal or trailing spaces or the number of 
newlines.
* Again, to repeat, Markua text formatting is supported, but Markua resources
are NOT supported inside a verbatim resource.

Now, while the whitespace is preserved, it is rendered using either a
proportional font or a monospaced one based on the value of the `monospaced`
attribute.

The `verbatim` resource should have newlines added by the author to ensure that
automatic line wrapping is not relied upon. Markua Processors may wrap lines to
ensure that all the text is visible on a page, and *may* add continuation
characters (like the backslash `\` character) in the output to indicate that a
line has been automatically wrapped by the Markua Processor. Obviously, seeing a
continuation character is in poetry is terrible, so the author should consider
this a mistake that needs to be fixed by manually line wrapping.

Verbatim resources can be a local, web or inline resource, just like any other
resource, and the same resource syntax applies to Markua resources as to all
other resources.

There are three main use cases for a Markua resource:

1. Inserting a specific `class` of resource, such as a `poem`.
2. Inserting boilerplate, such as a legal disclaimer, which may be needed to be
   used in many places and/or sourced from a third party.
3. Controlling the behavior of fonts or whitespace for specific types of
   text, such as poetry.

The following are the supported attributes for verbatim resources, in addition
to the `class`, `format`, `title` and `type` attributes which all resources
support.

`monospaced`
: `true` or `false`. The default is `false`. If `true`, the Markua Processor
must use a monospaced font to output the text in the resource. If false, the
Markua Processor may use whatever font (proportional or monospaced) it is
configured to use to output Markua text. The value of `true` can be useful for
certain types of poetry, for example.

Note that the standard `class` attribute should be used with a `verbatim` type
resource to define a specific `class` of resource on verbatim resources which
have titles, such as a `poem`. A List of Poems is a lot more poetic than a List
of Verbatims.

Also note that the standard resource `format` attribute for a `verbatim` type
resource must always be overridden to `verbatim` by a Markua Processor.
Similarly, if a `format` of `verbatim` is specified, this must set the `type`
to `verbatim`.

### Local Verbatim resources

Local and web resources with a `.text` extension default to being treated as a
`verbatim` type of resource.

~~~
Here's a Shakespearean sonnet, which has a type of `verbatim` and a class of
`poem`:

{type: verbatim, class: poem}
![](sonnet130.txt "Sonnet 130")

Here's a Shakespearean sonnet, which has a class of `poem` and will default to
the `{type: verbatim}` because of the `.text` extension:

{class: poem}
![](sonnet130.text "Sonnet 130")

Here's a Shakespearean sonnet, which has no class specified and which will
default to the `{type: verbatim}` because of the `.text` extension:

![](sonnet130.text "Sonnet 130")

Here's some legal boilerplate, which has no class specified and which will
default to the `{type: verbatim}` because of the `.text` extension:

![](tos.text "Terms and Conditions")

Here's an E. E. Cummings poem:

{type: verbatim, class: poem, monospaced: true}
![](iwillbe.txt "I Will Be")

Here's an E. E. Cummings poem which has no class specified and which will
default to the `{type: verbatim}` because of the `.text` extension:

{monospaced: true}
![](iwillbe.text "I Will Be")
~~~

### Web Verbatim resources

Web Markua resources function identically to how local Markua resources work,
including the significance of file extensions. The only difference is that the
files are on the web.

~~~
Here's a Shakespearean sonnet, which has a type of `verbatim` and a class of
`poem`:

{type: verbatim, class: poem}
![](https://markua.com/sonnet130.txt "Sonnet 130")

Here's a Shakespearean sonnet, which has a class of `poem` and will default to
the `{type: verbatim}` because of the `.text` extension:

{class: poem}
![](https://markua.com/sonnet130.text "Sonnet 130")

Here's a Shakespearean sonnet, which has no class specified and which will
default to the `{type: verbatim}` because of the `.text` extension:

![](https://markua.com/sonnet130.text "Sonnet 130")

Here's some legal boilerplate, which has no class specified and which will
default to the `{type: verbatim}` because of the `.text` extension:

![](https://markua.com/tos.text "Terms and Conditions")

Here's an E. E. Cummings poem:

{type: verbatim, class: poem, monospaced: true}
![](https://markua.com/iwillbe.txt "I Will Be")

Here's an E. E. Cummings poem which has no class specified and which will
default to the `{type: verbatim}` because of the `.text` extension:

{monospaced: true}
![](https://markua.com/iwillbe.text "I Will Be")
~~~

### Inline Verbatim resources

Inline verbatim resources are the easiest way to insert verbatim resources.

These work just like fenced code blocks, discussed earlier, but with a
`verbatim` format. (Recall from above that a `verbatim` format causes the `type`
to also be `verbatim`.)

Here's what this looks like with backticks:

~~~
Some paragraph.

```verbatim
I grant I never saw a goddess go;
My mistress when she walks treads on the ground.
    And yet, by heaven, I think my love as rare
    As any she belied with false compare.
```

Some paragraph.
~~~

You can also use tildes:

```
Some paragraph.

~~~verbatim
I grant I never saw a goddess go;
My mistress when she walks treads on the ground.
    And yet, by heaven, I think my love as rare
    As any she belied with false compare.
~~~

Some paragraph.
```

You can also use an attribute list:

~~~
Some paragraph.

{type: verbatim}
```
I grant I never saw a goddess go;
My mistress when she walks treads on the ground.
    And yet, by heaven, I think my love as rare
    As any she belied with false compare.
```

Some paragraph.
~~~

But the best choice is to use the `&` syntactic sugar, which means
`{type: verbatim}`:

~~~
Some paragraph.

```&
I grant I never saw a goddess go;
My mistress when she walks treads on the ground.
    And yet, by heaven, I think my love as rare
    As any she belied with false compare.
```

Some paragraph.
~~~

To summarize, what a `verbatim` resource type means is...

1. whitespace is preserved verbatim
2. no automatic indenting is done
3. basic Markua text formatting (bold, italic, etc.) is applied
4. Markua resources (e.g. tables, images, etc.) CANNOT be nested inside it
5. you can add `{monospaced: true | false}` (default `false`)
6. it can be local, web, or inline resource
7. the `format` is verbatim, which can be inferred from a `.text` extension
8. the normal rules apply for the `class` attribute



### But what if I want every character taken literally?

If you want to type poetry where every character is taken exactly literally,
instead of being potentially interpreted as Markua formatting, you need to use
a code block with a format of text for that:

```````````````````````````````` example
```text
*this* isn't italic
and **this** is not bold
* * * cherry blossoms * * *
```
.
<pre><code>*this* isn't italic
and **this** is not bold
* * * cherry blossoms * * *
</code></pre>
````````````````````````````````

I'm no poet, but at least I know it.

</div>



<div class="extension">

## No HTML blocks (M)

As [discussed](#markua-s-one-key-subtraction-from-markdown-raw-html-is-removed-m-),
Markua does not support raw HTML. **All raw HTML is removed**. As a side-effect,
this means HTML comments can still be used, since they (like all HTML) will be
removed from the output.

</div>



## Link reference definitions

A [link reference definition](@)
consists of a [link label], optionally preceded by up to three spaces of
indentation, followed
by a colon (`:`), optional spaces or tabs (including up to one
[line ending]), a [link destination],
optional spaces or tabs (including up to one
[line ending]), and an optional [link
title], which if it is present must be separated
from the [link destination] by spaces or tabs.
No further character may occur.

A [link reference definition]
does not correspond to a structural element of a document.  Instead, it
defines a label which can be used in [reference links]
and reference-style [images] elsewhere in the document.  [Link
reference definitions] can come either before or after the links that use
them.

```````````````````````````````` example
[foo]: /url "title"

[foo]
.
<p><a href="/url" title="title">foo</a></p>
````````````````````````````````


```````````````````````````````` example
   [foo]: 
      /url  
           'the title'  

[foo]
.
<p><a href="/url" title="the title">foo</a></p>
````````````````````````````````


```````````````````````````````` example
[Foo*bar\]]:my_(url) 'title (with parens)'

[Foo*bar\]]
.
<p><a href="my_(url)" title="title (with parens)">Foo*bar]</a></p>
````````````````````````````````


```````````````````````````````` example
[Foo bar]:
<my url>
'title'

[Foo bar]
.
<p><a href="my%20url" title="title">Foo bar</a></p>
````````````````````````````````


The title may extend over multiple lines:

```````````````````````````````` example
[foo]: /url '
title
line1
line2
'

[foo]
.
<p><a href="/url" title="
title
line1
line2
">foo</a></p>
````````````````````````````````


However, it may not contain a [blank line]:

```````````````````````````````` example
[foo]: /url 'title

with blank line'

[foo]
.
<p>[foo]: /url 'title</p>
<p>with blank line'</p>
<p>[foo]</p>
````````````````````````````````


The title may be omitted:

```````````````````````````````` example
[foo]:
/url

[foo]
.
<p><a href="/url">foo</a></p>
````````````````````````````````


The link destination may not be omitted:

```````````````````````````````` example
[foo]:

[foo]
.
<p>[foo]:</p>
<p>[foo]</p>
````````````````````````````````

 However, an empty link destination may be specified using
 angle brackets:

```````````````````````````````` example
[foo]: <>

[foo]
.
<p><a href="">foo</a></p>
````````````````````````````````

The title must be separated from the link destination by
spaces or tabs:

```````````````````````````````` example
[foo]: <bar>(baz)

[foo]
.
<p>[foo]: <bar>(baz)</p>
<p>[foo]</p>
````````````````````````````````


Both title and destination can contain backslash escapes
and literal backslashes:

```````````````````````````````` example
[foo]: /url\bar\*baz "foo\"bar\baz"

[foo]
.
<p><a href="/url%5Cbar*baz" title="foo&quot;bar\baz">foo</a></p>
````````````````````````````````


A link can come before its corresponding definition:

```````````````````````````````` example
[foo]

[foo]: url
.
<p><a href="url">foo</a></p>
````````````````````````````````


If there are several matching definitions, the first one takes
precedence:

```````````````````````````````` example
[foo]

[foo]: first
[foo]: second
.
<p><a href="first">foo</a></p>
````````````````````````````````


As noted in the section on [Links], matching of labels is
case-insensitive (see [matches]).

```````````````````````````````` example
[FOO]: /url

[Foo]
.
<p><a href="/url">Foo</a></p>
````````````````````````````````


```````````````````````````````` example
[ΑΓΩ]: /φου

[αγω]
.
<p><a href="/%CF%86%CE%BF%CF%85">αγω</a></p>
````````````````````````````````


Whether something is a [link reference definition] is
independent of whether the link reference it defines is
used in the document.  Thus, for example, the following
document contains just a link reference definition, and
no visible content:

```````````````````````````````` example
[foo]: /url
.
````````````````````````````````


Here is another one:

```````````````````````````````` example
[
foo
]: /url
bar
.
<p>bar</p>
````````````````````````````````


This is not a link reference definition, because there are
characters other than spaces or tabs after the title:

```````````````````````````````` example
[foo]: /url "title" ok
.
<p>[foo]: /url &quot;title&quot; ok</p>
````````````````````````````````


This is a link reference definition, but it has no title:

```````````````````````````````` example
[foo]: /url
"title" ok
.
<p>&quot;title&quot; ok</p>
````````````````````````````````


This is not a link reference definition, because it is indented
four spaces:

```````````````````````````````` example
    [foo]: /url "title"

[foo]
.
<pre><code>[foo]: /url &quot;title&quot;
</code></pre>
<p>[foo]</p>
````````````````````````````````


This is not a link reference definition, because it occurs inside
a code block:

```````````````````````````````` example
```
[foo]: /url
```

[foo]
.
<pre><code>[foo]: /url
</code></pre>
<p>[foo]</p>
````````````````````````````````


A [link reference definition] cannot interrupt a paragraph.

```````````````````````````````` example
Foo
[bar]: /baz

[bar]
.
<p>Foo
[bar]: /baz</p>
<p>[bar]</p>
````````````````````````````````


However, it can directly follow other block elements, such as headings
and thematic breaks, and it need not be followed by a blank line.

```````````````````````````````` example
# [Foo]
[foo]: /url
> bar
.
<h1><a href="/url">Foo</a></h1>
<blockquote>
<p>bar</p>
</blockquote>
````````````````````````````````

```````````````````````````````` example
[foo]: /url
bar
===
[foo]
.
<h1>bar</h1>
<p><a href="/url">foo</a></p>
````````````````````````````````

```````````````````````````````` example
[foo]: /url
===
[foo]
.
<p>===
<a href="/url">foo</a></p>
````````````````````````````````


Several [link reference definitions]
can occur one after another, without intervening blank lines.

```````````````````````````````` example
[foo]: /foo-url "foo"
[bar]: /bar-url
  "bar"
[baz]: /baz-url

[foo],
[bar],
[baz]
.
<p><a href="/foo-url" title="foo">foo</a>,
<a href="/bar-url" title="bar">bar</a>,
<a href="/baz-url">baz</a></p>
````````````````````````````````


[Link reference definitions] can occur
inside block containers, like lists and block quotations.  They
affect the entire document, not just the container in which they
are defined:

```````````````````````````````` example
[foo]

> [foo]: /url
.
<p><a href="/url">foo</a></p>
<blockquote>
</blockquote>
````````````````````````````````


## Paragraphs

A sequence of non-blank lines that cannot be interpreted as other
kinds of blocks forms a [paragraph](@).
The contents of the paragraph are the result of parsing the
paragraph's raw content as inlines.  The paragraph's raw content
is formed by concatenating the lines and removing initial and final
spaces or tabs.

A simple example with two paragraphs:

```````````````````````````````` example
aaa

bbb
.
<p>aaa</p>
<p>bbb</p>
````````````````````````````````


Paragraphs can contain multiple lines, but no blank lines:

```````````````````````````````` example
aaa
bbb

ccc
ddd
.
<p>aaa
bbb</p>
<p>ccc
ddd</p>
````````````````````````````````


Multiple blank lines between paragraphs have no effect:

```````````````````````````````` example
aaa


bbb
.
<p>aaa</p>
<p>bbb</p>
````````````````````````````````


Leading spaces or tabs are skipped:

```````````````````````````````` example
  aaa
 bbb
.
<p>aaa
bbb</p>
````````````````````````````````


Lines after the first may be indented any amount, since indented
code blocks cannot interrupt paragraphs.

```````````````````````````````` example
aaa
             bbb
                                       ccc
.
<p>aaa
bbb
ccc</p>
````````````````````````````````


However, the first line may be preceded by up to three spaces of indentation.
Four spaces of indentation is too many:

```````````````````````````````` example
   aaa
bbb
.
<p>aaa
bbb</p>
````````````````````````````````


```````````````````````````````` example
    aaa
bbb
.
<pre><code>aaa
</code></pre>
<p>bbb</p>
````````````````````````````````


Final spaces or tabs are stripped before inline parsing, so a paragraph
that ends with two or more spaces will not end with a [hard line
break]:

```````````````````````````````` example
aaa     
bbb     
.
<p>aaa<br />
bbb</p>
````````````````````````````````


## Blank lines

[Blank lines] between block-level elements are ignored,
except for the role they play in determining whether a [list]
is [tight] or [loose].

Blank lines at the beginning and end of the document are also ignored.

```````````````````````````````` example
  

aaa
  

# aaa

  
.
<p>aaa</p>
<h1>aaa</h1>
````````````````````````````````



<div class="extension">

## Tables (GFM)

GFM enables the `table` extension, where an additional leaf block type is
available.

A [table](@) is an arrangement of data with rows and columns, consisting of a
single header row, a [delimiter row] separating the header from the data, and
zero or more data rows.

Each row consists of cells containing arbitrary text, in which [inlines] are
parsed, separated by pipes (`|`).  A leading and trailing pipe is also
recommended for clarity of reading, and if there's otherwise parsing ambiguity.
Spaces between pipes and cell content are trimmed.  Block-level elements cannot
be inserted in a table.

The [delimiter row](@) consists of cells whose only content are hyphens (`-`),
and optionally, a leading or trailing colon (`:`), or both, to indicate left,
right, or center alignment respectively.

```````````````````````````````` example
| foo | bar |
| --- | --- |
| baz | bim |
.
<table>
<thead>
<tr>
<th>foo</th>
<th>bar</th>
</tr>
</thead>
<tbody>
<tr>
<td>baz</td>
<td>bim</td>
</tr>
</tbody>
</table>
````````````````````````````````

Cells in one column don't need to match length, though it's easier to read if
they are. Likewise, use of leading and trailing pipes may be inconsistent:

```````````````````````````````` example
| abc | defghi |
:-: | -----------:
bar | baz
.
<table>
<thead>
<tr>
<th align="center">abc</th>
<th align="right">defghi</th>
</tr>
</thead>
<tbody>
<tr>
<td align="center">bar</td>
<td align="right">baz</td>
</tr>
</tbody>
</table>
````````````````````````````````

Include a pipe in a cell's content by escaping it, including inside other
inline spans:

```````````````````````````````` example
| f\|oo  |
| ------ |
| b `\|` az |
| b **\|** im |
.
<table>
<thead>
<tr>
<th>f|oo</th>
</tr>
</thead>
<tbody>
<tr>
<td>b <code>|</code> az</td>
</tr>
<tr>
<td>b <strong>|</strong> im</td>
</tr>
</tbody>
</table>
````````````````````````````````

The table is broken at the first empty line, or beginning of another
block-level structure:

```````````````````````````````` example
| abc | def |
| --- | --- |
| bar | baz |
> bar
.
<table>
<thead>
<tr>
<th>abc</th>
<th>def</th>
</tr>
</thead>
<tbody>
<tr>
<td>bar</td>
<td>baz</td>
</tr>
</tbody>
</table>
<blockquote>
<p>bar</p>
</blockquote>
````````````````````````````````

```````````````````````````````` example
| abc | def |
| --- | --- |
| bar | baz |
bar

bar
.
<table>
<thead>
<tr>
<th>abc</th>
<th>def</th>
</tr>
</thead>
<tbody>
<tr>
<td>bar</td>
<td>baz</td>
</tr>
<tr>
<td>bar</td>
<td></td>
</tr>
</tbody>
</table>
<p>bar</p>
````````````````````````````````

The header row must match the [delimiter row] in the number of cells.  If not,
a table will not be recognized:

```````````````````````````````` example
| abc | def |
| --- |
| bar |
.
<p>| abc | def |
| --- |
| bar |</p>
````````````````````````````````

The remainder of the table's rows may vary in the number of cells.  If there
are a number of cells fewer than the number of cells in the header row, empty
cells are inserted.  If there are greater, the excess is ignored:

```````````````````````````````` example
| abc | def |
| --- | --- |
| bar |
| bar | baz | boo |
.
<table>
<thead>
<tr>
<th>abc</th>
<th>def</th>
</tr>
</thead>
<tbody>
<tr>
<td>bar</td>
<td></td>
</tr>
<tr>
<td>bar</td>
<td>baz</td>
</tr>
</tbody>
</table>
````````````````````````````````

If there are no rows in the body, no `<tbody>` is generated in HTML output:

```````````````````````````````` example
| abc | def |
| --- | --- |
.
<table>
<thead>
<tr>
<th>abc</th>
<th>def</th>
</tr>
</thead>
</table>
````````````````````````````````

</div>



<div class="extension">

## Table resources and CSV-format tables (M)

Tables can be formatted either as Markua tables or, for tables which are local
or web resources, as CSV-format tables. (CSV-format tables are not supported as
inline resources, since they would result in too many false-positives.)

The type and format of a table can be specified in an attribute list as follows:

```
{type: table, format: markua}
![](some_markua_table.txt)

{type: table, format: csv}
![](some_markua_table.txt)
```

The default type of a `csv` format resource is table, so the second example
could also be written like this:

```
{format: csv}
![](some_markua_table.txt)
```

However, the `.csv` file extension can also be used to provide the default
format of `csv`, so this can also be written like this:

```
![](some_markua_table.csv)
```

Also, if no format is specified but the type is specified to `table`, the
default is to assume a format of `markua`. So, the first example from above
could also be written like this:

```
{type: table}
![](some_markua_table.txt)
```

As shown in the Tables section, the syntax to insert a table in Markua is
the identical syntax to that used by GFM. (That section is unchanged from the
GFM spec, which is built on top of the CommonMark spec.)

Just as Markua reinterprets Markdown concepts like images as being resources,
Markua also reinterprets GFM tables as being a resource. This is important for
two reasons, in the case of tables:

1. Resources can have attribute lists.
2. Resources can be inline, local or web resources.

So, Markua supports attribute list on tables, and tables which are inline,
local or web resources:

* Inline means "in the document"; all tables in GFM are inline.
* Local means in the resources folder in a separate file, which can be helpful
  for keeping large tables from cluttering up your manuscript.
* Web means at some http or https URL.

Both local and web resources can be very useful for including data sourced from
an external data source, as well as making them easier to keep correct.

The following are the supported attributes for table resources, in
addition to the `class`, `format`, `title` and `type` attributes which all
resources support:

`align`
: The `align` can be `left`, `right` or `middle`. The default is `middle`.
This works the same way it does for images. Note that it applies to the position
of the entire table, not to the cells within it. In terms of the specific values
of `align`, a Markua Processor must interpret `left` as "on the left side of the
page", `right` as "on the right side of the page" and `middle` as "in the middle
of the content area of the page, respecting margins" in all cases. Finally, note
that `inside` and `outside` are not supported for `align`, and that a Markua
Processor may interpret `center` and/or `centre` as aliases for `middle`.
(The value of `middle` was chosen because of the `center` vs. `centre` spelling
issue.)

`columns`
: Sometimes it's desirable to only show part of a table resource defined in an
external file. The `rows` and `columns` attributes let you accomplish this.
The `columns` attribute can take a comma-separated list of values or ranges,
such as `2, 4, 7..9`, which would ensure that columns 2, 4, 7, 8 and 9 were
shown.

`column-widths`
: The column widths are a whitespace-separated list of numbers (integers and/or
floats) and/or * symbols, from left to right, as a percentage of the total table
width. In this attribute value, * means for the column to use the remaining
space, equally divided between it and any other column with the * attribute.
Some examples are: `{column-widths: "10% 30% * 10%"}`,
`{column-widths: "10% * 40% *"}`, `{column-widths: "10% 30% * 12.5%"}`,
`{column-widths: "95% * *"}`. The numbers used for the column-widths percentages
must sum to exactly 100 (if only numbers are used), or to less than 100 (if
there are any `*`s used). Every specified value must be at least 1, and every
`*` must compute to at least 1. The number of values (numbers or `*`s) must
match the number of columns. Like with the width attribute, the percentage sign
(`%`) is required, to make it absolutely clear that these are not measurements
in pixels or points.

`footer`
: `true`, `false` or `dynamic`. The default is `dynamic` for `markua` format
tables, and `false` for `csv` format tables. If `dynamic`, the existence of a
footer is determined by whether the content of the table contains a footer.
For a `csv` format table, `true` means that the last row is treated as a footer
row, and `false` means that the last row is not treated as a footer row.

`format`
: The `format` can be either `markua` (the default) or `csv`. The `csv` format
can ony be used with local or web resources, to reduce false positives.
CSV-format table resources are discussed in the next section.

`header`
: `true`, `false` or `dynamic`. The default is `dynamic` for `markua` format
tables, and `true` for `csv` format tables. If `dynamic`, the existence of a
header is determined by whether the content of the table contains a header.
For a `csv` format table, `true` means that the first row is treated as a header
row, and `false` means that the first row is not treated as a header row.

`rows`
: Sometimes it's desirable to only show part of a table resource defined in an
external file. The `rows` and `columns` attributes let you accomplish this.
The `rows` attribute can take a comma-separated list of values or ranges, such
as `1, 5, 13..18, 22`, which would ensure that rows 1, 5, 13, 14, 15, 16, 17,
18 and 22 were shown.

`width`
: The width of the table, in percentage of page content area width (respecting
margins). This is specified as an number (integer or float) between 1 and 100
followed by a percentage sign (%). The quotes are optional. So, you can say
`{width: "70%"}`, `{width: 70%}`, `{width: "70.5%"}` or `{width: 70.5%}`.

The examples below assume that a table which looks like the following (from the
GFM table example above) is defined in a file called `census.tbl` (real census
data would be too long for a good example):

```````````````````````````````` example
| foo | bar |
| --- | --- |
| baz | bim |
.
<table>
<thead>
<tr>
<th>foo</th>
<th>bar</th>
</tr>
</thead>
<tbody>
<tr>
<td>baz</td>
<td>bim</td>
</tr>
</tbody>
</table>
````````````````````````````````

With this table above defined in a file which is available at a local
resource location, the following example results:

```````````````````````````````` example
![](census.tbl "Canadian Census Data")
.
<figure>
  <table>
  <thead>
  <tr>
  <th>foo</th>
  <th>bar</th>
  </tr>
  </thead>
  <tbody>
  <tr>
  <td>baz</td>
  <td>bim</td>
  </tr>
  </tbody>
  </table>
  <figcaption>Canadian Census Data</figcaption>
</figure>
````````````````````````````````

With this table above defined in a file which is available at a given web
resource location, the identical example results:

```````````````````````````````` example
![](https://markua.com/census.tbl "Canadian Census Data")
.
<figure>
  <table>
  <thead>
  <tr>
  <th>foo</th>
  <th>bar</th>
  </tr>
  </thead>
  <tbody>
  <tr>
  <td>baz</td>
  <td>bim</td>
  </tr>
  </tbody>
  </table>
  <figcaption>Canadian Census Data</figcaption>
</figure>
````````````````````````````````

Table resources should be defined in a file with a `.tbl` extension. If the
extension is different, then there **must** be a `type: table` attribute in the
attribute list.

In the interest of brevity, some other examples of usage are below.

With the `.tbl` extension, the `table` type is inferred:

```
{title: "Canadian Census Data"}
![](census.tbl)

![](census.tbl "Canadian Census Data")

![Canadian Census Data](census.tbl)

{title: "Canadian Census Data", column-widths: "30% *"}
![](census.tbl)

{column-widths: "30% *"}
![](census.tbl "Canadian Census Data")

{column-widths: "30% *"}
![Canadian Census Data](census.tbl)
```

With a non-`.tbl` extension, the `table` type must be specified:

```
{type: table}
![](census.txt "Canadian Census Data")

{type: table, title: "Canadian Census Data"}
![](census.txt)

{type: table, title: "Canadian Census Data", column-widths: "30% *"}
![](census.txt)
```

Recall that the resource syntax is as follows:

```
{key: value, comma: separated, optional: attribute_list}
![optional alt text](resource_path_or_url "Optional Figure Title")
```

Note that there is some flexibility in the title attribute. If the `alt-title`
document setting is `text` or `all` (the default) , then the alt text can be
used as the title as well. If the `alt-title` is `none`, then it cannot.
The `alt-title` document setting is discussed
[in great detail here](#the-alt-title-document-setting-m-).

So, assuming a value of `text` or `all` for a document setting, this also works:

```
![Canadian Census Data](census.tbl)
```

(This is arguably the nicest looking version, and since tables are a text
resource there are no accessibility concerns about there being no alt text.)

This syntax can be used with an attribute list, of course:

```
{type: table}
![Canadian Census Data](census.txt)
```

Finally, if there is both alt text and a title attribute, the alt text is
clobbered by the title:

```
{align: middle, width: 80%, title: "Canadian Census Data"}
![some alt text which gets clobbered by the title](census.tbl)
```

As discussed above, tables (i.e. resources of type `table`) which are local or
web resources can have a `csv` format, instead of the default `markua` format.

For a `csv`-format table, the existence of a header and/or footer row cannot be
inferred from the data, and must be specified in an attribute list. The default
value of `header` is `true`, and the default value of `footer` is `false`.

Also, as discussed, a `csv` table can have `rows` and `columns`
attributes, for easily presenting a subset of a larger CSV. If `rows` and
`columns` are used with `header` of true, the header should be displayed by the
Markua Processor.

Examples:

```
{type: table, format: csv}
![Canadian Census Data](census.csv)

{type: table, format: csv, title: "Canadian Census Data"}
![](census.csv)

{type: table, format: csv}
![](census.csv "Canadian Census Data")

{type: table, format: csv, header: true}
![Canadian Census Data](census.csv)

{type: table, format: csv, header: true, footer: true}
![Canadian Census Data](census.csv)

{type: table, format: csv, header: true, footer: true, rows: 1..5, columns: 2,3}
![Canadian Census Data](census.csv)
```

</div>



# Container blocks

A [container block](#container-blocks) is a block that has other
blocks as its contents.  There are two basic kinds of container blocks:
[block quotes] and [list items].
[Lists] are meta-containers for [list items].

We define the syntax for container blocks recursively.  The general
form of the definition is:

> If X is a sequence of blocks, then the result of
> transforming X in such-and-such a way is a container of type Y
> with these blocks as its content.

So, we explain what counts as a block quote or list item by explaining
how these can be *generated* from their contents. This should suffice
to define the syntax, although it does not give a recipe for *parsing*
these constructions.  (A recipe is provided below in the section entitled
[A parsing strategy](#appendix-a-parsing-strategy).)

## Block quotes

A [block quote marker](@),
optionally preceded by up to three spaces of indentation,
consists of (a) the character `>` together with a following space of
indentation, or (b) a single character `>` not followed by a space of
indentation.

The following rules define [block quotes]:

1.  **Basic case.**  If a string of lines *Ls* constitute a sequence
    of blocks *Bs*, then the result of prepending a [block quote
    marker] to the beginning of each line in *Ls*
    is a [block quote](#block-quotes) containing *Bs*.

2.  **Laziness.**  If a string of lines *Ls* constitute a [block
    quote](#block-quotes) with contents *Bs*, then the result of deleting
    the initial [block quote marker] from one or
    more lines in which the next character other than a space or tab after the
    [block quote marker] is [paragraph continuation
    text] is a block quote with *Bs* as its content.
    [Paragraph continuation text](@) is text
    that will be parsed as part of the content of a paragraph, but does
    not occur at the beginning of the paragraph.

3.  **Consecutiveness.**  A document cannot contain two [block
    quotes] in a row unless there is a [blank line] between them.

Nothing else counts as a [block quote](#block-quotes).

Here is a simple example:

```````````````````````````````` example
> # Foo
> bar
> baz
.
<blockquote>
<h1>Foo</h1>
<p>bar
baz</p>
</blockquote>
````````````````````````````````


The space or tab after the `>` characters can be omitted:

```````````````````````````````` example
># Foo
>bar
> baz
.
<blockquote>
<h1>Foo</h1>
<p>bar
baz</p>
</blockquote>
````````````````````````````````


The `>` characters can be preceded by up to three spaces of indentation:

```````````````````````````````` example
   > # Foo
   > bar
 > baz
.
<blockquote>
<h1>Foo</h1>
<p>bar
baz</p>
</blockquote>
````````````````````````````````


Four spaces of indentation is too many:

```````````````````````````````` example
    > # Foo
    > bar
    > baz
.
<pre><code>&gt; # Foo
&gt; bar
&gt; baz
</code></pre>
````````````````````````````````


The Laziness clause allows us to omit the `>` before
[paragraph continuation text]:

```````````````````````````````` example
> # Foo
> bar
baz
.
<blockquote>
<h1>Foo</h1>
<p>bar
baz</p>
</blockquote>
````````````````````````````````


A block quote can contain some lazy and some non-lazy
continuation lines:

```````````````````````````````` example
> bar
baz
> foo
.
<blockquote>
<p>bar
baz
foo</p>
</blockquote>
````````````````````````````````


Laziness only applies to lines that would have been continuations of
paragraphs had they been prepended with [block quote markers].
For example, the `> ` cannot be omitted in the second line of

``` markdown
> foo
> ---
```

without changing the meaning:

```````````````````````````````` example
> foo
---
.
<blockquote>
<p>foo</p>
</blockquote>
<hr />
````````````````````````````````


Similarly, if we omit the `> ` in the second line of

``` markdown
> - foo
> - bar
```

then the block quote ends after the first line:

```````````````````````````````` example
> - foo
- bar
.
<blockquote>
<ul>
<li>foo</li>
</ul>
</blockquote>
<ul>
<li>bar</li>
</ul>
````````````````````````````````


For the same reason, we can't omit the `> ` in front of
subsequent lines of an indented or fenced code block:

```````````````````````````````` example
>     foo
    bar
.
<blockquote>
<pre><code>foo
</code></pre>
</blockquote>
<pre><code>bar
</code></pre>
````````````````````````````````


```````````````````````````````` example
> ```
foo
```
.
<blockquote>
<pre><code></code></pre>
</blockquote>
<p>foo</p>
<pre><code></code></pre>
````````````````````````````````


Note that in the following case, we have a [lazy
continuation line]:

```````````````````````````````` example
> foo
    - bar
.
<blockquote>
<p>foo
- bar</p>
</blockquote>
````````````````````````````````


To see why, note that in

```markdown
> foo
>     - bar
```

the `- bar` is indented too far to start a list, and can't
be an indented code block because indented code blocks cannot
interrupt paragraphs, so it is [paragraph continuation text].

A block quote can be empty:

```````````````````````````````` example
>
.
<blockquote>
</blockquote>
````````````````````````````````


```````````````````````````````` example
>
>  
> 
.
<blockquote>
</blockquote>
````````````````````````````````


A block quote can have initial or final blank lines:

```````````````````````````````` example
>
> foo
>  
.
<blockquote>
<p>foo</p>
</blockquote>
````````````````````````````````


A blank line always separates block quotes:

```````````````````````````````` example
> foo

> bar
.
<blockquote>
<p>foo</p>
</blockquote>
<blockquote>
<p>bar</p>
</blockquote>
````````````````````````````````


(Most current Markdown implementations, including John Gruber's
original `Markdown.pl`, will parse this example as a single block quote
with two paragraphs.  But it seems better to allow the author to decide
whether two block quotes or one are wanted.)

Consecutiveness means that if we put these block quotes together,
we get a single block quote:

```````````````````````````````` example
> foo
> bar
.
<blockquote>
<p>foo
bar</p>
</blockquote>
````````````````````````````````


To get a block quote with two paragraphs, use:

```````````````````````````````` example
> foo
>
> bar
.
<blockquote>
<p>foo</p>
<p>bar</p>
</blockquote>
````````````````````````````````


Block quotes can interrupt paragraphs:

```````````````````````````````` example
foo
> bar
.
<p>foo</p>
<blockquote>
<p>bar</p>
</blockquote>
````````````````````````````````


In general, blank lines are not needed before or after block
quotes:

```````````````````````````````` example
> aaa
***
> bbb
.
<blockquote>
<p>aaa</p>
</blockquote>
<hr />
<blockquote>
<p>bbb</p>
</blockquote>
````````````````````````````````


However, because of laziness, a blank line is needed between
a block quote and a following paragraph:

```````````````````````````````` example
> bar
baz
.
<blockquote>
<p>bar
baz</p>
</blockquote>
````````````````````````````````


```````````````````````````````` example
> bar

baz
.
<blockquote>
<p>bar</p>
</blockquote>
<p>baz</p>
````````````````````````````````


```````````````````````````````` example
> bar
>
baz
.
<blockquote>
<p>bar</p>
</blockquote>
<p>baz</p>
````````````````````````````````


It is a consequence of the Laziness rule that any number
of initial `>`s may be omitted on a continuation line of a
nested block quote:

```````````````````````````````` example
> > > foo
bar
.
<blockquote>
<blockquote>
<blockquote>
<p>foo
bar</p>
</blockquote>
</blockquote>
</blockquote>
````````````````````````````````


```````````````````````````````` example
>>> foo
> bar
>>baz
.
<blockquote>
<blockquote>
<blockquote>
<p>foo
bar
baz</p>
</blockquote>
</blockquote>
</blockquote>
````````````````````````````````


When including an indented code block in a block quote,
remember that the [block quote marker] includes
both the `>` and a following space of indentation.  So *five spaces* are needed
after the `>`:

```````````````````````````````` example
>     code

>    not code
.
<blockquote>
<pre><code>code
</code></pre>
</blockquote>
<blockquote>
<p>not code</p>
</blockquote>
````````````````````````````````



<div class="extension">

## Block quotes with curly braces (M)

Block quotes in Markua are created in one of two ways:

1. By prefacing lines with `> `, i.e. a greater than character followed by a
space. This was shown above.
2. By wrapping the blockquote in `{blockquote}` ... `{/blockquote}`

Option #1 is preferable for short quotes; option #2 is easier on authors for
really long quotes.

Like figures and tables, blockquotes can be inserted in the middle of a
paragraph or as a sibling of it.

These are the two ways to make block quotes in Markua:

```````````````````````````````` example
This is the first paragraph.

> This is a blockquote.
>
> You saw this above.

This is the second paragraph.

{blockquote}
This is also a blockquote.

It is good for longer quotes.
{/blockquote}

This is the third paragraph.
.
<p>This is the first paragraph.</p>
<blockquote>
<p>This is a blockquote.</p>
<p>You saw this above.</p>
</blockquote>
<p>This is the second paragraph.</p>
<blockquote>
<p>This is also a blockquote.</p>
<p>It is good for longer quotes.</p>
</blockquote>
<p>This is the third paragraph.</p>
````````````````````````````````

If a blockquote contains headings, these headings may be formatted by a Markua
Processor differently than normal headings. Finally, if a Markua Processor is
automatically generating a [Table of Contents](#toc) from chapter and section
headings, any headings inside blockquotes should be ignored.

</div>



## List items

A [list marker](@) is a
[bullet list marker] or an [ordered list marker].

A [bullet list marker](@)
is a `-`, `+`, or `*` character.

An [ordered list marker](@)
is a sequence of 1--9 arabic digits (`0-9`), followed by either a
`.` character or a `)` character.  (The reason for the length
limit is that with 10 digits we start seeing integer overflows
in some browsers.)

The following rules define [list items]:

1.  **Basic case.**  If a sequence of lines *Ls* constitute a sequence of
    blocks *Bs* starting with a character other than a space or tab, and *M* is
    a list marker of width *W* followed by 1 ≤ *N* ≤ 4 spaces of indentation,
    then the result of prepending *M* and the following spaces to the first line
    of *Ls*, and indenting subsequent lines of *Ls* by *W + N* spaces, is a
    list item with *Bs* as its contents.  The type of the list item
    (bullet or ordered) is determined by the type of its list marker.
    If the list item is ordered, then it is also assigned a start
    number, based on the ordered list marker.

    Exceptions:

    1. When the first list item in a [list] interrupts
       a paragraph---that is, when it starts on a line that would
       otherwise count as [paragraph continuation text]---then (a)
       the lines *Ls* must not begin with a blank line, and (b) if
       the list item is ordered, the start number must be 1.
    2. If any line is a [thematic break][thematic breaks] then
       that line is not a list item.

For example, let *Ls* be the lines

```````````````````````````````` example
A paragraph
with two lines.

    indented code

> A block quote.
.
<p>A paragraph
with two lines.</p>
<pre><code>indented code
</code></pre>
<blockquote>
<p>A block quote.</p>
</blockquote>
````````````````````````````````


And let *M* be the marker `1.`, and *N* = 2.  Then rule #1 says
that the following is an ordered list item with start number 1,
and the same contents as *Ls*:

```````````````````````````````` example
1.  A paragraph
    with two lines.

        indented code

    > A block quote.
.
<ol>
<li>
<p>A paragraph
with two lines.</p>
<pre><code>indented code
</code></pre>
<blockquote>
<p>A block quote.</p>
</blockquote>
</li>
</ol>
````````````````````````````````


The most important thing to notice is that the position of
the text after the list marker determines how much indentation
is needed in subsequent blocks in the list item.  If the list
marker takes up two spaces of indentation, and there are three spaces between
the list marker and the next character other than a space or tab, then blocks
must be indented five spaces in order to fall under the list
item.

Here are some examples showing how far content must be indented to be
put under the list item:

```````````````````````````````` example
- one

 two
.
<ul>
<li>one</li>
</ul>
<p>two</p>
````````````````````````````````


```````````````````````````````` example
- one

  two
.
<ul>
<li>
<p>one</p>
<p>two</p>
</li>
</ul>
````````````````````````````````


```````````````````````````````` example
 -    one

     two
.
<ul>
<li>one</li>
</ul>
<pre><code> two
</code></pre>
````````````````````````````````


```````````````````````````````` example
 -    one

      two
.
<ul>
<li>
<p>one</p>
<p>two</p>
</li>
</ul>
````````````````````````````````


It is tempting to think of this in terms of columns:  the continuation
blocks must be indented at least to the column of the first character other than
a space or tab after the list marker.  However, that is not quite right.
The spaces of indentation after the list marker determine how much relative
indentation is needed.  Which column this indentation reaches will depend on
how the list item is embedded in other constructions, as shown by
this example:

```````````````````````````````` example
   > > 1.  one
>>
>>     two
.
<blockquote>
<blockquote>
<ol>
<li>
<p>one</p>
<p>two</p>
</li>
</ol>
</blockquote>
</blockquote>
````````````````````````````````


Here `two` occurs in the same column as the list marker `1.`,
but is actually contained in the list item, because there is
sufficient indentation after the last containing blockquote marker.

The converse is also possible.  In the following example, the word `two`
occurs far to the right of the initial text of the list item, `one`, but
it is not considered part of the list item, because it is not indented
far enough past the blockquote marker:

```````````````````````````````` example
>>- one
>>
  >  > two
.
<blockquote>
<blockquote>
<ul>
<li>one</li>
</ul>
<p>two</p>
</blockquote>
</blockquote>
````````````````````````````````


Note that at least one space or tab is needed between the list marker and
any following content, so these are not list items:

```````````````````````````````` example
-one

2.two
.
<p>-one</p>
<p>2.two</p>
````````````````````````````````


A list item may contain blocks that are separated by more than
one blank line.

```````````````````````````````` example
- foo


  bar
.
<ul>
<li>
<p>foo</p>
<p>bar</p>
</li>
</ul>
````````````````````````````````


A list item may contain any kind of block:

```````````````````````````````` example
1.  foo

    ```
    bar
    ```

    baz

    > bam
.
<ol>
<li>
<p>foo</p>
<pre><code>bar
</code></pre>
<p>baz</p>
<blockquote>
<p>bam</p>
</blockquote>
</li>
</ol>
````````````````````````````````


A list item that contains an indented code block will preserve
empty lines within the code block verbatim.

```````````````````````````````` example
- Foo

      bar


      baz
.
<ul>
<li>
<p>Foo</p>
<pre><code>bar


baz
</code></pre>
</li>
</ul>
````````````````````````````````

Note that ordered list start numbers must be nine digits or less:

```````````````````````````````` example
123456789. ok
.
<ol start="123456789">
<li>ok</li>
</ol>
````````````````````````````````


```````````````````````````````` example
1234567890. not ok
.
<p>1234567890. not ok</p>
````````````````````````````````


A start number may begin with 0s:

```````````````````````````````` example
0. ok
.
<ol start="0">
<li>ok</li>
</ol>
````````````````````````````````


```````````````````````````````` example
003. ok
.
<ol start="3">
<li>ok</li>
</ol>
````````````````````````````````


A start number may not be negative:

```````````````````````````````` example
-1. not ok
.
<p>-1. not ok</p>
````````````````````````````````



2.  **Item starting with indented code.**  If a sequence of lines *Ls*
    constitute a sequence of blocks *Bs* starting with an indented code
    block, and *M* is a list marker of width *W* followed by
    one space of indentation, then the result of prepending *M* and the
    following space to the first line of *Ls*, and indenting subsequent lines
    of *Ls* by *W + 1* spaces, is a list item with *Bs* as its contents.
    If a line is empty, then it need not be indented.  The type of the
    list item (bullet or ordered) is determined by the type of its list
    marker.  If the list item is ordered, then it is also assigned a
    start number, based on the ordered list marker.

An indented code block will have to be preceded by four spaces of indentation
beyond the edge of the region where text will be included in the list item.
In the following case that is 6 spaces:

```````````````````````````````` example
- foo

      bar
.
<ul>
<li>
<p>foo</p>
<pre><code>bar
</code></pre>
</li>
</ul>
````````````````````````````````


And in this case it is 11 spaces:

```````````````````````````````` example
  10.  foo

           bar
.
<ol start="10">
<li>
<p>foo</p>
<pre><code>bar
</code></pre>
</li>
</ol>
````````````````````````````````


If the *first* block in the list item is an indented code block,
then by rule #2, the contents must be preceded by *one* space of indentation
after the list marker:

```````````````````````````````` example
    indented code

paragraph

    more code
.
<pre><code>indented code
</code></pre>
<p>paragraph</p>
<pre><code>more code
</code></pre>
````````````````````````````````


```````````````````````````````` example
1.     indented code

   paragraph

       more code
.
<ol>
<li>
<pre><code>indented code
</code></pre>
<p>paragraph</p>
<pre><code>more code
</code></pre>
</li>
</ol>
````````````````````````````````


Note that an additional space of indentation is interpreted as space
inside the code block:

```````````````````````````````` example
1.      indented code

   paragraph

       more code
.
<ol>
<li>
<pre><code> indented code
</code></pre>
<p>paragraph</p>
<pre><code>more code
</code></pre>
</li>
</ol>
````````````````````````````````


Note that rules #1 and #2 only apply to two cases:  (a) cases
in which the lines to be included in a list item begin with a
character other than a space or tab, and (b) cases in which
they begin with an indented code
block.  In a case like the following, where the first block begins with
three spaces of indentation, the rules do not allow us to form a list item by
indenting the whole thing and prepending a list marker:

```````````````````````````````` example
   foo

bar
.
<p>foo</p>
<p>bar</p>
````````````````````````````````


```````````````````````````````` example
-    foo

  bar
.
<ul>
<li>foo</li>
</ul>
<p>bar</p>
````````````````````````````````


This is not a significant restriction, because when a block is preceded by up to
three spaces of indentation, the indentation can always be removed without
a change in interpretation, allowing rule #1 to be applied.  So, in
the above case:

```````````````````````````````` example
-  foo

   bar
.
<ul>
<li>
<p>foo</p>
<p>bar</p>
</li>
</ul>
````````````````````````````````


3.  **Item starting with a blank line.**  If a sequence of lines *Ls*
    starting with a single [blank line] constitute a (possibly empty)
    sequence of blocks *Bs*, and *M* is a list marker of width *W*,
    then the result of prepending *M* to the first line of *Ls*, and
    preceding subsequent lines of *Ls* by *W + 1* spaces of indentation, is a
    list item with *Bs* as its contents.
    If a line is empty, then it need not be indented.  The type of the
    list item (bullet or ordered) is determined by the type of its list
    marker.  If the list item is ordered, then it is also assigned a
    start number, based on the ordered list marker.

Here are some list items that start with a blank line but are not empty:

```````````````````````````````` example
-
  foo
-
  ```
  bar
  ```
-
      baz
.
<ul>
<li>foo</li>
<li>
<pre><code>bar
</code></pre>
</li>
<li>
<pre><code>baz
</code></pre>
</li>
</ul>
````````````````````````````````

When the list item starts with a blank line, the number of spaces
following the list marker doesn't change the required indentation:

```````````````````````````````` example
-   
  foo
.
<ul>
<li>foo</li>
</ul>
````````````````````````````````


A list item can begin with at most one blank line.
In the following example, `foo` is not part of the list
item:

```````````````````````````````` example
-

  foo
.
<ul>
<li></li>
</ul>
<p>foo</p>
````````````````````````````````


Here is an empty bullet list item:

```````````````````````````````` example
- foo
-
- bar
.
<ul>
<li>foo</li>
<li></li>
<li>bar</li>
</ul>
````````````````````````````````


It does not matter whether there are spaces or tabs following the [list marker]:

```````````````````````````````` example
- foo
-   
- bar
.
<ul>
<li>foo</li>
<li></li>
<li>bar</li>
</ul>
````````````````````````````````


Here is an empty ordered list item:

```````````````````````````````` example
1. foo
2.
3. bar
.
<ol>
<li>foo</li>
<li></li>
<li>bar</li>
</ol>
````````````````````````````````


A list may start or end with an empty list item:

```````````````````````````````` example
*
.
<ul>
<li></li>
</ul>
````````````````````````````````

However, an empty list item cannot interrupt a paragraph:

```````````````````````````````` example
foo
*

foo
1.
.
<p>foo
*</p>
<p>foo
1.</p>
````````````````````````````````


4.  **Indentation.**  If a sequence of lines *Ls* constitutes a list item
    according to rule #1, #2, or #3, then the result of preceding each line
    of *Ls* by up to three spaces of indentation (the same for each line) also
    constitutes a list item with the same contents and attributes.  If a line is
    empty, then it need not be indented.

Indented one space:

```````````````````````````````` example
 1.  A paragraph
     with two lines.

         indented code

     > A block quote.
.
<ol>
<li>
<p>A paragraph
with two lines.</p>
<pre><code>indented code
</code></pre>
<blockquote>
<p>A block quote.</p>
</blockquote>
</li>
</ol>
````````````````````````````````


Indented two spaces:

```````````````````````````````` example
  1.  A paragraph
      with two lines.

          indented code

      > A block quote.
.
<ol>
<li>
<p>A paragraph
with two lines.</p>
<pre><code>indented code
</code></pre>
<blockquote>
<p>A block quote.</p>
</blockquote>
</li>
</ol>
````````````````````````````````


Indented three spaces:

```````````````````````````````` example
   1.  A paragraph
       with two lines.

           indented code

       > A block quote.
.
<ol>
<li>
<p>A paragraph
with two lines.</p>
<pre><code>indented code
</code></pre>
<blockquote>
<p>A block quote.</p>
</blockquote>
</li>
</ol>
````````````````````````````````


Four spaces indent gives a code block:

```````````````````````````````` example
    1.  A paragraph
        with two lines.

            indented code

        > A block quote.
.
<pre><code>1.  A paragraph
    with two lines.

        indented code

    &gt; A block quote.
</code></pre>
````````````````````````````````



5.  **Laziness.**  If a string of lines *Ls* constitute a [list
    item](#list-items) with contents *Bs*, then the result of deleting
    some or all of the indentation from one or more lines in which the
    next character other than a space or tab after the indentation is
    [paragraph continuation text] is a
    list item with the same contents and attributes.  The unindented
    lines are called
    [lazy continuation line](@)s.

Here is an example with [lazy continuation lines]:

```````````````````````````````` example
  1.  A paragraph
with two lines.

          indented code

      > A block quote.
.
<ol>
<li>
<p>A paragraph
with two lines.</p>
<pre><code>indented code
</code></pre>
<blockquote>
<p>A block quote.</p>
</blockquote>
</li>
</ol>
````````````````````````````````


Indentation can be partially deleted:

```````````````````````````````` example
  1.  A paragraph
    with two lines.
.
<ol>
<li>A paragraph
with two lines.</li>
</ol>
````````````````````````````````


These examples show how laziness can work in nested structures:

```````````````````````````````` example
> 1. > Blockquote
continued here.
.
<blockquote>
<ol>
<li>
<blockquote>
<p>Blockquote
continued here.</p>
</blockquote>
</li>
</ol>
</blockquote>
````````````````````````````````


```````````````````````````````` example
> 1. > Blockquote
> continued here.
.
<blockquote>
<ol>
<li>
<blockquote>
<p>Blockquote
continued here.</p>
</blockquote>
</li>
</ol>
</blockquote>
````````````````````````````````



6.  **That's all.** Nothing that is not counted as a list item by rules
    #1--5 counts as a [list item](#list-items).

The rules for sublists follow from the general rules
[above][List items].  A sublist must be indented the same number
of spaces of indentation a paragraph would need to be in order to be included
in the list item.

So, in this case we need two spaces indent:

```````````````````````````````` example
- foo
  - bar
    - baz
      - boo
.
<ul>
<li>foo
<ul>
<li>bar
<ul>
<li>baz
<ul>
<li>boo</li>
</ul>
</li>
</ul>
</li>
</ul>
</li>
</ul>
````````````````````````````````


One is not enough:

```````````````````````````````` example
- foo
 - bar
  - baz
   - boo
.
<ul>
<li>foo</li>
<li>bar</li>
<li>baz</li>
<li>boo</li>
</ul>
````````````````````````````````


Here we need four, because the list marker is wider:

```````````````````````````````` example
10) foo
    - bar
.
<ol start="10">
<li>foo
<ul>
<li>bar</li>
</ul>
</li>
</ol>
````````````````````````````````


Three is not enough:

```````````````````````````````` example
10) foo
   - bar
.
<ol start="10">
<li>foo</li>
</ol>
<ul>
<li>bar</li>
</ul>
````````````````````````````````


A list may be the first block in a list item:

```````````````````````````````` example
- - foo
.
<ul>
<li>
<ul>
<li>foo</li>
</ul>
</li>
</ul>
````````````````````````````````


```````````````````````````````` example
1. - 2. foo
.
<ol>
<li>
<ul>
<li>
<ol start="2">
<li>foo</li>
</ol>
</li>
</ul>
</li>
</ol>
````````````````````````````````


A list item can contain a heading:

```````````````````````````````` example
- # Foo
- Bar
  ---
  baz
.
<ul>
<li>
<h1>Foo</h1>
</li>
<li>
<h2>Bar</h2>
baz</li>
</ul>
````````````````````````````````


### Motivation

John Gruber's Markdown spec says the following about list items:

1. "List markers typically start at the left margin, but may be indented
   by up to three spaces. List markers must be followed by one or more
   spaces or a tab."

2. "To make lists look nice, you can wrap items with hanging indents....
   But if you don't want to, you don't have to."

3. "List items may consist of multiple paragraphs. Each subsequent
   paragraph in a list item must be indented by either 4 spaces or one
   tab."

4. "It looks nice if you indent every line of the subsequent paragraphs,
   but here again, Markdown will allow you to be lazy."

5. "To put a blockquote within a list item, the blockquote's `>`
   delimiters need to be indented."

6. "To put a code block within a list item, the code block needs to be
   indented twice — 8 spaces or two tabs."

These rules specify that a paragraph under a list item must be indented
four spaces (presumably, from the left margin, rather than the start of
the list marker, but this is not said), and that code under a list item
must be indented eight spaces instead of the usual four.  They also say
that a block quote must be indented, but not by how much; however, the
example given has four spaces indentation.  Although nothing is said
about other kinds of block-level content, it is certainly reasonable to
infer that *all* block elements under a list item, including other
lists, must be indented four spaces.  This principle has been called the
*four-space rule*.

The four-space rule is clear and principled, and if the reference
implementation `Markdown.pl` had followed it, it probably would have
become the standard.  However, `Markdown.pl` allowed paragraphs and
sublists to start with only two spaces indentation, at least on the
outer level.  Worse, its behavior was inconsistent: a sublist of an
outer-level list needed two spaces indentation, but a sublist of this
sublist needed three spaces.  It is not surprising, then, that different
implementations of Markdown have developed very different rules for
determining what comes under a list item.  (Pandoc and python-Markdown,
for example, stuck with Gruber's syntax description and the four-space
rule, while discount, redcarpet, marked, PHP Markdown, and others
followed `Markdown.pl`'s behavior more closely.)

Unfortunately, given the divergences between implementations, there
is no way to give a spec for list items that will be guaranteed not
to break any existing documents.  However, the spec given here should
correctly handle lists formatted with either the four-space rule or
the more forgiving `Markdown.pl` behavior, provided they are laid out
in a way that is natural for a human to read.

The strategy here is to let the width and indentation of the list marker
determine the indentation necessary for blocks to fall under the list
item, rather than having a fixed and arbitrary number.  The writer can
think of the body of the list item as a unit which gets indented to the
right enough to fit the list marker (and any indentation on the list
marker).  (The laziness rule, #5, then allows continuation lines to be
unindented if needed.)

This rule is superior, we claim, to any rule requiring a fixed level of
indentation from the margin.  The four-space rule is clear but
unnatural. It is quite unintuitive that

``` markdown
- foo

  bar

  - baz
```

should be parsed as two lists with an intervening paragraph,

``` html
<ul>
<li>foo</li>
</ul>
<p>bar</p>
<ul>
<li>baz</li>
</ul>
```

as the four-space rule demands, rather than a single list,

``` html
<ul>
<li>
<p>foo</p>
<p>bar</p>
<ul>
<li>baz</li>
</ul>
</li>
</ul>
```

The choice of four spaces is arbitrary.  It can be learned, but it is
not likely to be guessed, and it trips up beginners regularly.

Would it help to adopt a two-space rule?  The problem is that such
a rule, together with the rule allowing up to three spaces of indentation for
the initial list marker, allows text that is indented *less than* the
original list marker to be included in the list item. For example,
`Markdown.pl` parses

``` markdown
   - one

  two
```

as a single list item, with `two` a continuation paragraph:

``` html
<ul>
<li>
<p>one</p>
<p>two</p>
</li>
</ul>
```

and similarly

``` markdown
>   - one
>
>  two
```

as

``` html
<blockquote>
<ul>
<li>
<p>one</p>
<p>two</p>
</li>
</ul>
</blockquote>
```

This is extremely unintuitive.

Rather than requiring a fixed indent from the margin, we could require
a fixed indent (say, two spaces, or even one space) from the list marker (which
may itself be indented).  This proposal would remove the last anomaly
discussed.  Unlike the spec presented above, it would count the following
as a list item with a subparagraph, even though the paragraph `bar`
is not indented as far as the first paragraph `foo`:

``` markdown
 10. foo

   bar  
```

Arguably this text does read like a list item with `bar` as a subparagraph,
which may count in favor of the proposal.  However, on this proposal indented
code would have to be indented six spaces after the list marker.  And this
would break a lot of existing Markdown, which has the pattern:

``` markdown
1.  foo

        indented code
```

where the code is indented eight spaces.  The spec above, by contrast, will
parse this text as expected, since the code block's indentation is measured
from the beginning of `foo`.

The one case that needs special treatment is a list item that *starts*
with indented code.  How much indentation is required in that case, since
we don't have a "first paragraph" to measure from?  Rule #2 simply stipulates
that in such cases, we require one space indentation from the list marker
(and then the normal four spaces for the indented code).  This will match the
four-space rule in cases where the list marker plus its initial indentation
takes four spaces (a common case), but diverge in other cases.

## Lists

A [list](@) is a sequence of one or more
list items [of the same type].  The list items
may be separated by any number of blank lines.

Two list items are [of the same type](@)
if they begin with a [list marker] of the same type.
Two list markers are of the
same type if (a) they are bullet list markers using the same character
(`-`, `+`, or `*`) or (b) they are ordered list numbers with the same
delimiter (either `.` or `)`).

A list is an [ordered list](@)
if its constituent list items begin with
[ordered list markers], and a
[bullet list](@) if its constituent list
items begin with [bullet list markers].

The [start number](@)
of an [ordered list] is determined by the list number of
its initial list item.  The numbers of subsequent list items are
disregarded.

A list is [loose](@) if any of its constituent
list items are separated by blank lines, or if any of its constituent
list items directly contain two block-level elements with a blank line
between them.  Otherwise a list is [tight](@).
(The difference in HTML output is that paragraphs in a loose list are
wrapped in `<p>` tags, while paragraphs in a tight list are not.)

Changing the bullet or ordered list delimiter starts a new list:

```````````````````````````````` example
- foo
- bar
+ baz
.
<ul>
<li>foo</li>
<li>bar</li>
</ul>
<ul>
<li>baz</li>
</ul>
````````````````````````````````


```````````````````````````````` example
1. foo
2. bar
3) baz
.
<ol>
<li>foo</li>
<li>bar</li>
</ol>
<ol start="3">
<li>baz</li>
</ol>
````````````````````````````````


In CommonMark, a list can interrupt a paragraph. That is,
no blank line is needed to separate a paragraph from a following
list:

```````````````````````````````` example
Foo
- bar
- baz
.
<p>Foo</p>
<ul>
<li>bar</li>
<li>baz</li>
</ul>
````````````````````````````````

`Markdown.pl` does not allow this, through fear of triggering a list
via a numeral in a hard-wrapped line:

``` markdown
The number of windows in my house is
14.  The number of doors is 6.
```

Oddly, though, `Markdown.pl` *does* allow a blockquote to
interrupt a paragraph, even though the same considerations might
apply.

In CommonMark, we do allow lists to interrupt paragraphs, for
two reasons.  First, it is natural and not uncommon for people
to start lists without blank lines:

``` markdown
I need to buy
- new shoes
- a coat
- a plane ticket
```

Second, we are attracted to a

> [principle of uniformity](@):
> if a chunk of text has a certain
> meaning, it will continue to have the same meaning when put into a
> container block (such as a list item or blockquote).

(Indeed, the spec for [list items] and [block quotes] presupposes
this principle.) This principle implies that if

``` markdown
  * I need to buy
    - new shoes
    - a coat
    - a plane ticket
```

is a list item containing a paragraph followed by a nested sublist,
as all Markdown implementations agree it is (though the paragraph
may be rendered without `<p>` tags, since the list is "tight"),
then

``` markdown
I need to buy
- new shoes
- a coat
- a plane ticket
```

by itself should be a paragraph followed by a nested sublist.

Since it is well established Markdown practice to allow lists to
interrupt paragraphs inside list items, the [principle of
uniformity] requires us to allow this outside list items as
well.  ([reStructuredText](https://docutils.sourceforge.net/rst.html)
takes a different approach, requiring blank lines before lists
even inside other list items.)

In order to solve the problem of unwanted lists in paragraphs with
hard-wrapped numerals, we allow only lists starting with `1` to
interrupt paragraphs.  Thus,

```````````````````````````````` example
The number of windows in my house is
14.  The number of doors is 6.
.
<p>The number of windows in my house is
14.  The number of doors is 6.</p>
````````````````````````````````

We may still get an unintended result in cases like

```````````````````````````````` example
The number of windows in my house is
1.  The number of doors is 6.
.
<p>The number of windows in my house is</p>
<ol>
<li>The number of doors is 6.</li>
</ol>
````````````````````````````````

but this rule should prevent most spurious list captures.

There can be any number of blank lines between items:

```````````````````````````````` example
- foo

- bar


- baz
.
<ul>
<li>
<p>foo</p>
</li>
<li>
<p>bar</p>
</li>
<li>
<p>baz</p>
</li>
</ul>
````````````````````````````````

```````````````````````````````` example
- foo
  - bar
    - baz


      bim
.
<ul>
<li>foo
<ul>
<li>bar
<ul>
<li>
<p>baz</p>
<p>bim</p>
</li>
</ul>
</li>
</ul>
</li>
</ul>
````````````````````````````````


To separate consecutive lists of the same type, or to separate a
list from an indented code block that would otherwise be parsed
as a subparagraph of the final list item, you can insert a blank HTML
comment:

```````````````````````````````` example
- foo
- bar

<!-- -->

- baz
- bim
.
<ul>
<li>foo</li>
<li>bar</li>
</ul>
<!-- -->
<ul>
<li>baz</li>
<li>bim</li>
</ul>
````````````````````````````````


```````````````````````````````` example
-   foo

    notcode

-   foo

<!-- -->

    code
.
<ul>
<li>
<p>foo</p>
<p>notcode</p>
</li>
<li>
<p>foo</p>
</li>
</ul>
<!-- -->
<pre><code>code
</code></pre>
````````````````````````````````


List items need not be indented to the same level.  The following
list items will be treated as items at the same list level,
since none is indented enough to belong to the previous list
item:

```````````````````````````````` example
- a
 - b
  - c
   - d
  - e
 - f
- g
.
<ul>
<li>a</li>
<li>b</li>
<li>c</li>
<li>d</li>
<li>e</li>
<li>f</li>
<li>g</li>
</ul>
````````````````````````````````


```````````````````````````````` example
1. a

  2. b

   3. c
.
<ol>
<li>
<p>a</p>
</li>
<li>
<p>b</p>
</li>
<li>
<p>c</p>
</li>
</ol>
````````````````````````````````

Note, however, that list items may not be preceded by more than
three spaces of indentation.  Here `- e` is treated as a paragraph continuation
line, because it is indented more than three spaces:

```````````````````````````````` example
- a
 - b
  - c
   - d
    - e
.
<ul>
<li>a</li>
<li>b</li>
<li>c</li>
<li>d
- e</li>
</ul>
````````````````````````````````

And here, `3. c` is treated as in indented code block,
because it is indented four spaces and preceded by a
blank line.

```````````````````````````````` example
1. a

  2. b

    3. c
.
<ol>
<li>
<p>a</p>
</li>
<li>
<p>b</p>
</li>
</ol>
<pre><code>3. c
</code></pre>
````````````````````````````````


This is a loose list, because there is a blank line between
two of the list items:

```````````````````````````````` example
- a
- b

- c
.
<ul>
<li>
<p>a</p>
</li>
<li>
<p>b</p>
</li>
<li>
<p>c</p>
</li>
</ul>
````````````````````````````````


So is this, with a empty second item:

```````````````````````````````` example
* a
*

* c
.
<ul>
<li>
<p>a</p>
</li>
<li></li>
<li>
<p>c</p>
</li>
</ul>
````````````````````````````````


These are loose lists, even though there are no blank lines between the items,
because one of the items directly contains two block-level elements
with a blank line between them:

```````````````````````````````` example
- a
- b

  c
- d
.
<ul>
<li>
<p>a</p>
</li>
<li>
<p>b</p>
<p>c</p>
</li>
<li>
<p>d</p>
</li>
</ul>
````````````````````````````````


```````````````````````````````` example
- a
- b

  [ref]: /url
- d
.
<ul>
<li>
<p>a</p>
</li>
<li>
<p>b</p>
</li>
<li>
<p>d</p>
</li>
</ul>
````````````````````````````````


This is a tight list, because the blank lines are in a code block:

```````````````````````````````` example
- a
- ```
  b


  ```
- c
.
<ul>
<li>a</li>
<li>
<pre><code>b


</code></pre>
</li>
<li>c</li>
</ul>
````````````````````````````````


This is a tight list, because the blank line is between two
paragraphs of a sublist.  So the sublist is loose while
the outer list is tight:

```````````````````````````````` example
- a
  - b

    c
- d
.
<ul>
<li>a
<ul>
<li>
<p>b</p>
<p>c</p>
</li>
</ul>
</li>
<li>d</li>
</ul>
````````````````````````````````


This is a tight list, because the blank line is inside the
block quote:

```````````````````````````````` example
* a
  > b
  >
* c
.
<ul>
<li>a
<blockquote>
<p>b</p>
</blockquote>
</li>
<li>c</li>
</ul>
````````````````````````````````


This list is tight, because the consecutive block elements
are not separated by blank lines:

```````````````````````````````` example
- a
  > b
  ```
  c
  ```
- d
.
<ul>
<li>a
<blockquote>
<p>b</p>
</blockquote>
<pre><code>c
</code></pre>
</li>
<li>d</li>
</ul>
````````````````````````````````


A single-paragraph list is tight:

```````````````````````````````` example
- a
.
<ul>
<li>a</li>
</ul>
````````````````````````````````


```````````````````````````````` example
- a
  - b
.
<ul>
<li>a
<ul>
<li>b</li>
</ul>
</li>
</ul>
````````````````````````````````


This list is loose, because of the blank line between the
two block elements in the list item:

```````````````````````````````` example
1. ```
   foo
   ```

   bar
.
<ol>
<li>
<pre><code>foo
</code></pre>
<p>bar</p>
</li>
</ol>
````````````````````````````````


Here the outer list is loose, the inner list tight:

```````````````````````````````` example
* foo
  * bar

  baz
.
<ul>
<li>
<p>foo</p>
<ul>
<li>bar</li>
</ul>
<p>baz</p>
</li>
</ul>
````````````````````````````````


```````````````````````````````` example
- a
  - b
  - c

- d
  - e
  - f
.
<ul>
<li>
<p>a</p>
<ul>
<li>b</li>
<li>c</li>
</ul>
</li>
<li>
<p>d</p>
<ul>
<li>e</li>
<li>f</li>
</ul>
</li>
</ul>
````````````````````````````````



<div class="extension">

## List attributes (M)

By default, Markua keeps the behavior of lists to be identical to that in
Markdown, as described by John Gruber and specified by CommonMark.
The enhancements that Markua makes to lists are purely additive: if you use any
CommonMark-compliant list in a Markua document, it will work exactly as it would
in CommonMark.

Now, lists in CommonMark have some major limitations. Here are two of the
biggest ones:

1. You can't output descending order lists.
2. You can't output letters or Roman numerals for labels in an ordered list.

This list gets rendered as 3,4,5 (not 3,2,1), since Markdown does not have a way
of specifying that a list is descending, and it does not infer it from the order
by default:

```````````````````````````````` example
before

3. foo
2. bar
1. baz

after
.
<p>before</p>
<ol start="3">
<li>foo</li>
<li>bar</li>
<li>baz</li>
</ol>
<p>after</p>
````````````````````````````````

This looks like a list, but it is in fact a paragraph. There are no break tags
after the foo, bar and baz, just soft breaks:

```````````````````````````````` example
before

a. foo
b. bar
c. baz

after
.
<p>before</p>
<p>a. foo
b. bar
c. baz</p>
<p>after</p>
````````````````````````````````

Again, both of these behaviors are as defined in the CommonMark spec.

Now, Markua can't change this by default, without being incompatibile with
Markdown.

Also, CommonMark allows [slight inconsistency](#example-295) in the amount of
indentation of list items. I understand why, but this allows for documents which
are incomprehensible and ugly. However, Markua must keep compatibility with
CommonMark lists, so this is allowed.

So what to do?

Markua adds the **list attributes** extension.

Adding attributes to lists allows them to output all the needed types of list
from the limited list support in CommonMark, without introducing breaking
incompatibilities to CommonMark lists.

It is certainly possible to define a better list syntax than Markdown's: I have
previously done exactly that, specifying a syntax for complex lists which was
superior to Markdown's lists. Furthermore, Leanpub even implemented support for
it! However, this syntax introduced incompatibilites with Markdown, and the
tradeoff was not worth it. Markua's list enhancements must be purely additive
to CommonMark lists, not incompatible with them in any way. They cannot
introduce any breaking changes to existing CommonMark lists, or authors will be
justifiably unhappy.

The way the list attributes extension works is simple: it gives lists the
ability to have an **attribute list**, much like how resources can have an
attribute list. (Lists are not resources, however. That would not just be
overkill, it would result in incompatibilites.)

### Supported Attributes for Lists

The following are the supported attributes for lists.

`reversed`
: This determines whether the list is in ascending or descending order. Legal
values are `true` or `false`. The default value is `false`, for ascending.
Any value other than `true` is interpreted as `false`.

`type`
: The type of the list marker to use. Valid choices are `1`, `A`, `a`, `I` or
`i`. `A` and `a` mean uppercase or lowercase letters; `I` or `i` mean uppercase
or lowercase Roman numerals. Note that lists are NOT resources, so it is fine
to use `type` for the list marker: there is no `{type: list}` resource, so there
is no naming conflict.

`delimiter`
: The type of the list delimiter to use, in output formats which allow custom
list delimiters. Valid choices are `"."` or `")"`. The default value is inferred
by the delimiter used: if you use periods `.` to delimit the list, then the
delimiter is `.`; if you use closing parenthesis `)` to delimit the list, then
the delimiter is `)`. Note that the use of a closing parenthesis as a delimiter
is not supported in all output formats. Specifically, HTML only supports using
`.` for a list delimiter, so `)` is not supported in HTML. And since EPUB is
based on HTML, EPUB should not be expected to support the use of a right
parenthesis for a delimiter. However, PDF and LaTeX are not subject to these
limitations, and should support both `.` and `)` as list delimiters.

The use of these attributes are shown in the following examples.

### Lists with Attributes: Good Style

This list will be numbered a, b, c, and will have the delimiter of `.`:

```````````````````````````````` example
before

{type: a}
1. foo
2. bar
3. baz

after
.
<p>before</p>
<ol type="a">
<li>foo</li>
<li>bar</li>
<li>baz</li>
</ol>
<p>after</p>
````````````````````````````````

This list will be numbered a, b, c, and will have the delimiter of `.`:

```````````````````````````````` example
before

{type: a, delimiter: "."}
1. foo
2. bar
3. baz

after
.
<p>before</p>
<ol type="a">
<li>foo</li>
<li>bar</li>
<li>baz</li>
</ol>
<p>after</p>
````````````````````````````````

This list will be numbered a, b, c, and will have the delimiter of `)` in
output formats that support it (e.g. in PDF but not in HTML):

```````````````````````````````` example
before

{type: a}
1) foo
2) bar
3) baz

after
.
<p>before</p>
<ol type="a">
<li>foo</li>
<li>bar</li>
<li>baz</li>
</ol>
<p>after</p>
````````````````````````````````

This list will be numbered a, b, c, and will have the delimiter of `)` in
output formats that support it (e.g. in PDF but not in HTML):

```````````````````````````````` example
before

{type: a, delimiter: ")"}
1) foo
2) bar
3) baz

after
.
<p>before</p>
<ol type="a">
<li>foo</li>
<li>bar</li>
<li>baz</li>
</ol>
<p>after</p>
````````````````````````````````

This list will be numbered A, B, C:

```````````````````````````````` example
before

{type: A}
1. foo
2. bar
3. baz

after
.
<p>before</p>
<ol type="A">
<li>foo</li>
<li>bar</li>
<li>baz</li>
</ol>
<p>after</p>
````````````````````````````````

This list will be numbered c, d, e:

```````````````````````````````` example
before

{type: a}
3. foo
4. bar
5. baz

after
.
<p>before</p>
<ol type="a" start="3">
<li>foo</li>
<li>bar</li>
<li>baz</li>
</ol>
<p>after</p>
````````````````````````````````

This list will be numbered i, ii, iii:

```````````````````````````````` example
before

{type: i}
1. foo
2. bar
3. baz

after
.
<p>before</p>
<ol type="i">
<li>foo</li>
<li>bar</li>
<li>baz</li>
</ol>
<p>after</p>
````````````````````````````````

This list will be numbered I, II, III:

```````````````````````````````` example
before

{type: I}
1. foo
2. bar
3. baz

after
.
<p>before</p>
<ol type="i">
<li>foo</li>
<li>bar</li>
<li>baz</li>
</ol>
<p>after</p>
````````````````````````````````

This list will be numbered iii, iv, v:

```````````````````````````````` example
before

{type: i}
3. foo
4. bar
5. baz

after
.
<p>before</p>
<ol type="i" start="3">
<li>foo</li>
<li>bar</li>
<li>baz</li>
</ol>
<p>after</p>
````````````````````````````````

This list will be numbered 3, 2, 1:

```````````````````````````````` example
before

{reversed: true}
3. foo
2. bar
1. baz

after
.
<p>before</p>
<ol start="3" reversed="true">
<li>foo</li>
<li>bar</li>
<li>baz</li>
</ol>
<p>after</p>
````````````````````````````````

This list will be numbered c, b, a:

```````````````````````````````` example
before

{type: a, reversed: true}
3. foo
2. bar
1. baz

after
.
<p>before</p>
<ol start="3" type="a" reversed="true">
<li>foo</li>
<li>bar</li>
<li>baz</li>
</ol>
<p>after</p>
````````````````````````````````

This list will be numbered iii, ii, i:

```````````````````````````````` example
before

{type: i, reversed: true}
3. foo
2. bar
1. baz

after
.
<p>before</p>
<ol start="3" type="i" reversed="true">
<li>foo</li>
<li>bar</li>
<li>baz</li>
</ol>
<p>after</p>
````````````````````````````````

### Nested Lists without Attributes

By default, nested lists which do not have attributes function as though they
have a `type` attribute whose value is determined by the context in which the
list is found.

By default, in order, the delimiter chosen from outermost to innermost nested
numbered lists are:

`1., a., i., 1), a), i)`

* The first level numbered list is numbered using Arabic numerals followed by a
  period (`1.`, `2.`, `3.`, `4.`, ...)
* The second level nested list is numbered using lowercase letters followed by a
  period (`a.`, `b.`, `c.`, `d.`, ...)
* The third level nested list is numbered using Roman numerals followed by a
  period (`i.`, `ii.`, `iii.`, `iv.`, ...)
* The fourth level numbered list is numbered using Arabic numerals followed by a
  right parenthesis (`1)`, `2)`, `3)`, `4)`, ...)
* The fifth level nested list is numbered using lowercase letters followed by a
  right parenthesis (`a)`, `b)`, `c)`, `d)`, ...)
* The sixth level nested list is numbered using Roman numerals followed by a
  right parenthesis (`i)`, `ii)`, `iii)`, `iv)`, ...)

```````````````````````````````` example
before

1. lorem
   1. foo
      1. apple
         1. Gala
         2. Spartan
         3. McIntosh
      2. orange
      3. banana
   2. bar
   3. baz
2. ipsum
3. dolor

after
.
<p>before</p>
<ol>
<li>lorem
<ol type="a">
<li>foo
<ol type="i">
<li>apple
<ol>
<li>Gala</li>
<li>Spartan</li>
<li>McIntosh</li>
</ol>
</li>
<li>orange</li>
<li>banana</li>
</ol>
</li>
<li>bar</li>
<li>baz</li>
</ol>
</li>
<li>ipsum</li>
<li>dolor</li>
</ol>
<p>after</p>
````````````````````````````````

This nested list will be rendered somewhat like this:

~~~
before

1. lorem
   a. foo
        i. apple
           1) Gala
           2) Spartan
           3) McIntosh
       ii. orange
      iii. banana
   b. bar
   c. baz
2. ipsum
3. dolor

after
~~~

Note that as discussed earlier in section 8.5.1, Supported Attributes for
Lists, the support of the right parenthesis for a delimiter varies based on
the output format. In cases where it is not supported, a period will be used
instead.

### Nested Lists with Attributes

Everything which has been said about attribute lists on lists applies to nested
lists as well. This example shows a nested list of the form 1, a, i, * (the
final nested list is an unordered list, not an ordered list):

```````````````````````````````` example
before

1. lorem
   {type: a}
   1. foo
      {type: i}
      1. apple
         * Gala
         * Spartan
         * McIntosh
      2. orange
      3. banana
   2. bar
   3. baz
2. ipsum
3. dolor

after
.
<p>before</p>
<ol>
<li>lorem
<ol type="a">
<li>foo
<ol type="i">
<li>apple
<ul>
<li>Gala</li>
<li>Spartan</li>
<li>McIntosh</li>
</ul>
</li>
<li>orange</li>
<li>banana</li>
</ol>
</li>
<li>bar</li>
<li>baz</li>
</ol>
</li>
<li>ipsum</li>
<li>dolor</li>
</ol>
<p>after</p>
````````````````````````````````

This nested list will be rendered somewhat like this:

~~~
before

1. lorem
   a. foo
        i. apple
           * Gala
           * Spartan
           * McIntosh
       ii. orange
      iii. banana
   b. bar
   c. baz
2. ipsum
3. dolor

after
~~~

### Lists with Attributes: Lazy Style

It is acceptable to just use the number of the first label, and rely on list
behavior.

This list will be numbered 1, 2, 3:

```````````````````````````````` example
before

1. foo
1. bar
1. baz

after
.
<p>before</p>
<ol>
<li>foo</li>
<li>bar</li>
<li>baz</li>
</ol>
<p>after</p>
````````````````````````````````

This list will be numbered 3, 4, 5:

```````````````````````````````` example
before

3. foo
3. bar
3. baz

after
.
<p>before</p>
<ol start="3">
<li>foo</li>
<li>bar</li>
<li>baz</li>
</ol>
<p>after</p>
````````````````````````````````

This list will be numbered c, d, e:

```````````````````````````````` example
before

{type: a}
3. foo
3. bar
3. baz

after
.
<p>before</p>
<ol start="3" type="a">
<li>foo</li>
<li>bar</li>
<li>baz</li>
</ol>
<p>after</p>
````````````````````````````````

This list will be numbered iii, iv, v:

```````````````````````````````` example
before

{type: i}
3. foo
3. bar
3. baz

after
.
<p>before</p>
<ol start="3" type="i">
<li>foo</li>
<li>bar</li>
<li>baz</li>
</ol>
<p>after</p>
````````````````````````````````

This list will be numbered 3, 2, 1:

```````````````````````````````` example
before

{reversed: true}
3. foo
3. bar
3. baz

after
.
<p>before</p>
<ol start="3" reversed="true">
<li>foo</li>
<li>bar</li>
<li>baz</li>
</ol>
<p>after</p>
````````````````````````````````

### Lists with Attributes: Redundant Style

This list will be numbered 1, 2, 3. This is just the default behavior made
explicit:

```````````````````````````````` example
before

{type: 1, reversed: false}
1. foo
2. bar
3. baz

after
.
<p>before</p>
<ol>
<li>foo</li>
<li>bar</li>
<li>baz</li>
</ol>
<p>after</p>
````````````````````````````````

### Lists with Attributes: Bad Style

This list will be numbered a, b, c, and will have the delimiter of `.`.
The reason that this is bad style is that the specified delimiter and the
inferred delimiter do not match:

```````````````````````````````` example
before

{type: a, delimiter: "."}
1) foo
2) bar
3) baz

after
.
<p>before</p>
<ol type="a">
<li>foo</li>
<li>bar</li>
<li>baz</li>
</ol>
<p>after</p>
````````````````````````````````

This list will be numbered a, b, c, and will have the delimiter of `)` in
output formats that support it (e.g. in PDF but not in HTML). The reason
that this is bad style is that the specified delimiter and the inferred
delimiter do not match:

```````````````````````````````` example
before

{type: a, delimiter: ")"}
1. foo
2. bar
3. baz

after
.
<p>before</p>
<ol type="a">
<li>foo</li>
<li>bar</li>
<li>baz</li>
</ol>
<p>after</p>
````````````````````````````````

This list will be numbered 3, 2, 1, not 3, 4, 5. Only the first number matters
since the reversed attribute is set:

```````````````````````````````` example
before

{reversed: true}
3. foo
4. bar
5. baz

after
.
<p>before</p>
<ol reversed="true">
<li>foo</li>
<li>bar</li>
<li>baz</li>
</ol>
<p>after</p>
````````````````````````````````

This list will be numbered 1, 0, -1, not 1, 2, 3. Only the first number matters
since the reversed attribute is set:

```````````````````````````````` example
before

{reversed: true}
1. foo
2. bar
3. baz

after
.
<p>before</p>
<ol reversed="true">
<li>foo</li>
<li>bar</li>
<li>baz</li>
</ol>
<p>after</p>
````````````````````````````````

This list will be numbered c, d, e, regardless of the order of the numbers:

```````````````````````````````` example
before

{type: a}
3. foo
2. bar
1. baz

after
.
<p>before</p>
<ol start="3" type="a">
<li>three</li>
<li>bar</li>
<li>one</li>
</ol>
<p>after</p>
````````````````````````````````

This list will be numbered 1, 0, -1:

```````````````````````````````` example
before

{reversed: true}
1. foo
1. bar
1. baz

after
.
<p>before</p>
<ol reversed="true" start="1">
<li>foo</li>
<li>bar</li>
<li>baz</li>
</ol>
<p>after</p>
````````````````````````````````

This list will be numbered i, 0, -1, since the Romans didn't have the concept of
zero or negative numbers (and thus neither do Roman numerals):

```````````````````````````````` example
before

{type: i, reversed: true}
1. foo
1. bar
1. baz

after
.
<p>before</p>
<ol reversed="true" start="1" type="i">
<li>foo</li>
<li>bar</li>
<li>baz</li>
</ol>
<p>after</p>
````````````````````````````````

This list will be numbered a, 0, -1, since there are no negative letters either:

```````````````````````````````` example
before

{type: a, reversed: true}
1. foo
1. bar
1. baz

after
.
<p>before</p>
<ol reversed="true" start="1" type="a">
<li>foo</li>
<li>bar</li>
<li>baz</li>
</ol>
<p>after</p>
````````````````````````````````

</div>



<div class="extension">

## Definition lists (M)

Definition lists are also supported in Markua. To define a definition list, use
the following syntax:

```````````````````````````````` example
term 1
: definition 1

term 2
: definition 2
.
<dl>
  <dt>term 1</dt>
  <dd>definition 1</dd>

  <dt>term 1</dt>
  <dd>definition 1</dd>
</dl>
````````````````````````````````

The *term* portion of a definition list item may only contain simple inline
content such as text, code spans, images (which are not figures), etc. It may
not contain resources or arbitrary block content such as figures. The
*definition* portion of a definition list item may contain arbitrary block
content.

There can be one to three spaces after the colon, or one tab.

Note that in HTML the terms produce `<dt>` elements (for "description term")
and the definitions produce `<dd>` elements (for "description details"). Since
it is not certain that the definition list contains definitions, and since it
is desirable for the syntax to remain simple, the `<dd>` element does not
contain a `<dfn>` element.

There can only be one term per definition, but there can be multiple definitions
for a term:

```````````````````````````````` example
term 1
: definition 1a
: definition 1b

term 2
: definition 2
.
<dl>
  <dt>term 1</dt>
  <dd>definition 1a</dd>
  <dd>definition 1b</dd>

  <dt>term 2</dt>
  <dd>definition 2</dd>
</dl>
````````````````````````````````

A single term definition list is a definition list, regardless of how many
definitions for the term exist:

```````````````````````````````` example
term
: definition
.
<dl>
  <dt>term</dt>
  <dd>definition</dd>
</dl>
````````````````````````````````

Finally, each definition list item can contain newlines and multiple paragraphs.
What you do here is indent the subsequent lines by the same amount of space as
the initial line.

(If you do not indent the subsequent lines, then you're ending the definition
list and just starting a new paragraph.)

```````````````````````````````` example
before

one
: foo
: bar

  baz

two
: foo

after
.
<p>before</p>
<dl>
  <dt>one</dt>
  <dd>foo</dd>
  <dd>
    <p>bar</p>
    <p>baz</p>
  </dd>

  <dt>two</dt>
  <dd>foo</dd>
</dl>
<p>after</p>
````````````````````````````````

Note that single line breaks within the definitions (the `<dd>` not `<dt>`)
are handled the same way as single line breaks within paragraphs, and are
subject to the `soft-breaks` document setting which defaults to `space` for
compatibility with Markdown.

```````````````````````````````` example
before

one
: lorem
  ipsum

  dolor

  sit amet

two
: foo

after
.
<p>before</p>
<dl>
  <dt>one</dt>
  <dd>
    <p>lorem
    ipsum</p>
    <p>dolor</p>
    <p>sit amet</p>
  </dd>

  <dt>two</dt>
  <dd>foo</dd>
</dl>
<p>after</p>
````````````````````````````````

Finally, with definition lists, you need to link to a specific definition,
not just to the list itself. This is useful to do, since in a document with many
definitions, it's helpful if the reader scrolls to the right spot or opens to
the right page.

To do this, just define a span id on the element itself, and then link to it.

```````````````````````````````` example
foo{#foo}
: This is foo.

bar{#bar}
: This is bar.
.
<dl>
  <dt><span id="foo">foo</span></dt>
  <dd>This is foo.</dd>

  <dt><span id="bar">bar</span></dt>
  <dd>This is bar.</dd>
</dl>
````````````````````````````````

</div>



<div class="extension">

## Asides (M)

Since Markua is for writing books and courses, including technical books and
courses, it needs not just a syntax for blockquotes--it also needs a syntax for
asides and for blurbs.

These syntaxes are very similar to the Markua syntax for blockquotes. Like
blockquotes, any headings inside asides or blurbs do not show up in a Table of
Contents (if one is present), and they can also be formatted differently by
Markua Processors.

We will consider asides first.

Asides are typically short or long notes in books which are tangential to the
main idea--sort of like footnotes, but in the body text itself. In technical
books, quite often they are formatted in a box. Asides can span multiple pages.

The syntaxes for asides are very similar to blockquotes:

1. By prefacing lines with `A> `, i.e. an `A`, then a greater than character
   (`>`), then a space.
2. By wrapping the aside in `{aside}` ... `{/aside}`

Option #1 is preferable for short asides; option #2 is easier on authors for
really long asides.

For consistency with blockquotes, asides can be siblings of paragraphs or nested
in them.

Here's a short aside:

```````````````````````````````` example
A> This is a short aside.
.
<aside>
<p>This is a short aside.</p>
</aside>
````````````````````````````````

Here's a longer aside, which also contains a heading:

```````````````````````````````` example
A> # A Longer Aside
A>
A> This is a longer aside.
A>
A> It can have multiple paragraphs.
.
<aside>
<h1>A Longer Aside</h1>
<p>This is a longer aside.</p>
<p>It can have multiple paragraphs.</p>
</aside>
````````````````````````````````

Here's the same longer aside using the `{aside}` syntax:

```````````````````````````````` example
{aside}
# A Longer Aside

This is a longer aside.

It can have multiple paragraphs.
{/aside}
.
<aside>
<h1>A Longer Aside</h1>
<p>This is a longer aside.</p>
<p>It can have multiple paragraphs.</p>
</aside>
````````````````````````````````

</div>



<div class="extension">

## Blurbs (M)

Blurbs are like asides, but shorter. A blurb is not intended to span multiple
pages of output.

The syntaxes for blurbs are very similar to asides:

1. By prefacing lines with `B> `, i.e. a `B`, then a greater than character
   (`>`), then a space.
2. By wrapping the blurb in `{blurb}` ... `{/blurb}`

Here's a short blurb:

```````````````````````````````` example
B> This is a short blurb.
.
<aside class="blurb">
<p>This is a short blurb.</p>
</aside>
````````````````````````````````

Here's a longer blurb, which also contains a heading:

```````````````````````````````` example
B> # A Longer Blurb
B>
B> This is a longer blurb.
B>
B> It can have multiple paragraphs.
.
<aside class="blurb">
<h1>A Longer Blurb</h1>
<p>This is a longer blurb.</p>
<p>It can have multiple paragraphs.</p>
</aside>
````````````````````````````````

Here's the same longer blurb using the `{blurb}` syntax:

```````````````````````````````` example
{blurb}
# A Longer Blurb

This is a longer blurb.

It can have multiple paragraphs.
{/blurb}
.
<aside class="blurb">
<h1>A Longer Blurb</h1>
<p>This is a longer blurb.</p>
<p>It can have multiple paragraphs.</p>
</aside>
````````````````````````````````

### Blurb classes

Blurbs also have support for an attribute list, which can contain a `class`
attribute.

Markua has its origin in authoring computer programming books. In computer
programming books, there are a number of blurb types which are a
de facto standard:

* `center`
* `discussion`
* `error`
* `information`
* `tip`
* `warning`

These blurb types can be set on a blurb as its `class` attribute. A Markua
Processor can optionally style these blurbs appropriately based on the class,
for example by adding custom icons for each class of blurb.

Here's how this looks with the `B> ` syntax:

```````````````````````````````` example
{class: warning}
B> This is a warning!
.
<aside class="blurb warning">
<p>This is a warning!</p>
</aside>
````````````````````````````````

Here's how this looks with the `{blurb}` syntax:

```````````````````````````````` example
{blurb, class: warning}
This is a warning!
{/blurb}
.
<aside class="blurb warning">
<p>This is a warning!</p>
</aside>
````````````````````````````````

The attribute list must either directly precede the `B>` with no blank line
between it and the `B>`, or it must be combined with the `{blurb}` block
opening. It is **NOT** legal Markua syntax to have an attribute list preceding a
`{blurb}` block opening like this:

~~~
{class: warning}
{blurb}
That is not legal Markua...
~~~


### Syntactic sugar for blurb classes

Having to constantly type `{class: warning}` in a computer programming book with
a number of warnings would get tedious, as would any of the other blurb classes
listed above.

So, Markua defines a standard shorthand syntax for these classes of blurbs. With
this syntax, you use a different letter than `B` in the `B>`, to create a blurb
with the appropriate `class`.

These are the syntactic sugar values you can use which have a heritage in
computer programming books:

`D>`
: `{class: discussion}`

`E>`
: `{class: error}`

`I>`
: `{class: information}`

`Q>`
: `{class: question}`

`T>`
: `{class: tip}`

`W>`
: `{class: warning}`

`X>`
: `{class: exercise}`

```````````````````````````````` example
D> This is a discussion blurb.

{class: discussion}
B> This is a discussion blurb.

{blurb, class: discussion}
This is a discussion blurb.
{/blurb}
.
<aside class="blurb discussion">
<p>This is a discussion blurb.</p>
</aside>

<aside class="blurb discussion">
<p>This is a discussion blurb.</p>
</aside>

<aside class="blurb discussion">
<p>This is a discussion blurb.</p>
</aside>
````````````````````````````````

```````````````````````````````` example
E> This is an error blurb.

{class: error}
B> This is an error blurb.

{blurb, class: error}
This is an error blurb.
{/blurb}
.
<aside class="blurb error">
<p>This is an error blurb.</p>
</aside>

<aside class="blurb error">
<p>This is an error blurb.</p>
</aside>

<aside class="blurb error">
<p>This is an error blurb.</p>
</aside>
````````````````````````````````

```````````````````````````````` example
X> This is an exercise blurb.

{class: exercise}
B> This is an exercise blurb.

{blurb, class: exercise}
This is an exercise blurb.
{/blurb}
.
<aside class="blurb exercise">
<p>This is an exercise blurb.</p>
</aside>

<aside class="blurb exercise">
<p>This is an exercise blurb.</p>
</aside>

<aside class="blurb exercise">
<p>This is an exercise blurb.</p>
</aside>
````````````````````````````````

```````````````````````````````` example
I> This is an information blurb.

{class: information}
B> This is an information blurb.

{blurb, class: information}
This is an information blurb.
{/blurb}
.
<aside class="blurb information">
<p>This is an information blurb.</p>
</aside>

<aside class="blurb information">
<p>This is an information blurb.</p>
</aside>

<aside class="blurb information">
<p>This is an information blurb.</p>
</aside>
````````````````````````````````

```````````````````````````````` example
Q> This is a question blurb.

{class: question}
B> This is a question blurb.

{blurb, class: question}
This is a question blurb.
{/blurb}
.
<aside class="blurb question">
<p>This is a question blurb.</p>
</aside>

<aside class="blurb question">
<p>This is a question blurb.</p>
</aside>

<aside class="blurb question">
<p>This is a question blurb.</p>
</aside>
````````````````````````````````

```````````````````````````````` example
T> This is a tip blurb.

{class: tip}
B> This is a tip blurb.

{blurb, class: tip}
This is a tip blurb.
{/blurb}
.
<aside class="blurb tip">
<p>This is a tip blurb.</p>
</aside>

<aside class="blurb tip">
<p>This is a tip blurb.</p>
</aside>

<aside class="blurb tip">
<p>This is a tip blurb.</p>
</aside>
````````````````````````````````

```````````````````````````````` example
W> This is a warning blurb.

{class: warning}
B> This is a warning blurb.

{blurb, class: warning}
This is a warning blurb.
{/blurb}
.
<aside class="blurb warning">
<p>This is a warning blurb.</p>
</aside>

<aside class="blurb warning">
<p>This is a warning blurb.</p>
</aside>

<aside class="blurb warning">
<p>This is a warning blurb.</p>
</aside>
````````````````````````````````

Note that `Q>` and `X>` are a bit confusing:

* `Q>` defines a blurb which is formatted like a question, but `{quiz}`
(discussed later) defines a quiz, and quizzes have actual numbered questions in
them. It is unfortunate that the words `quiz` and `question` both start with the
letter `Q`. Also, please note that the `question` blurb is not the same thing as
a question in a quiz.
* `X>` defines a blurb which is formatted like an exercise, but `{exercise}`
(discussed later) defines a structured exercise similar to a quiz. It is
unfortunate that the term "exercise" is used for both. However, picking a worse
word (such as, say, example) just to avoid any ambiguity would be worse.

Also note that nothing in this section defines what a Markua Processor must *do*
with the given class of blurb, in terms of formatting. Leanpub, for example, uses
it to add an appropriate icon from
[Font Awesome](https://fontawesome.com/search?m=free&o=r)
at the left of the blurb, but other Markua Processors are free to do something
different.

Finally, note that specifying a class in metadata overrides what the syntactic
sugar does, and is also an error:

```````````````````````````````` example
{class: tip}
W> This is a tip blurb, not a warning blurb.
.
<aside class="blurb tip">
<p>This is a tip blurb, not a warning blurb.</p>
</aside>
````````````````````````````````

### Using blurbs to center text

You can also use a blurb to center text.

The following two ways to do this are equivalent:

```````````````````````````````` example
C> This is a centered blurb.

{class: center}
B> This is a centered blurb.
.
<aside class="blurb center">
<p>This is a centered blurb.</p>
</aside>
<aside class="blurb center">
<p>This is a centered blurb.</p>
</aside>
````````````````````````````````

This is the only way to center text in Markua.

Unlike other blurb types which have their origin in technical books, centering
text has a wide range of uses. So, it could have been thought of as something
different than a blurb. However, in terms of its behavior and the way it's
inserted, centered text is a blurb--whether it's inserted via syntactic sugar or
via a class attribute on a normal blurb element.


### Blurb icons with extension attributes

Markua Processors must ignore any attributes which they do not understand.

Because of this, Markua attribute lists can contain any number of extension
attributes. An extension attribute is an attribute which is not defined in the
Markua specification, but which is understood by some specific Markua Processor.

As an example of an extension attribute, Leanpub adds an `icon` attribute to
blurbs. Markua does not specify that a blurb must support an `icon` attribute,
or what it would mean if it did. However, Leanpub understands an `icon`
attribute to reference an icon from Font Awesome. The value of this attribute
is assumed to be the name of an icon in Font Awesome, without the `fa-` prefix.
So, in any Markua Processor, you can do this:

~~~
{icon: car}
B> You can't spell carbon without it!

{icon: leanpub}
B> Yes, we're in Font Awesome!

{icon: github}
B> So is GitHub, of course. Unicorns.
~~~

In Leanpub, this will produce a nice icon of a car, using the Font Awesome icon.
In a Markua implementation that does not understand the icon attribute, nothing
will be generated for that attribute--it will be functionally equivalent to the
attribute not being present.

</div>



<div class="extension">

# Quizzes and exercises (M)

Markua was initially designed for the writing of books, but it has been extended
to support creating courses. For example, Leanpub authors can click a button to
create an online course, complete with automated marking, entirely from a Markua
document.

**To convert a Markua document from a book into a course, all you need to do is
add quizzes and exercises.**

Over the past decade, there has been a steady growth of interest in courses
delivered over the internet at massive scale. These courses consist of
essentially four things:

1. Reading material
2. Video or audio lectures
3. Exercises, with answers provided to the student
4. Quizzes, with answers used to automatically mark the quiz

It turns out the four things in this list all work perfectly in a Markua
document. So, not only can Markua be used to easily create a textbook which
includes video, audio, images and quizzes, it is also an amazingly simple and
flexible way of creating an online course. An online course is essentially just
a textbook which is executable, plus discussion forums and credentials. For
example, [Leanpub](https://leanpub.com) authors can click one button to create a
course, complete with automated marking for all the quizzes in the course,
entirely from one Markua document.

Quizzes and exercises are essentially the same. The only difference is that
quizzes are intended to be marked, and exercises are not. Because of their
similarities, they are discussed here together.

Quizzes or exercises in a textbook consist of two things:

1. Questions, typically in the chapter itself.
2. Answers, typically at the back of the book.

The questions in the chapter essentially are placed there like any other block
element, such as an aside or blurb. The answers are positioned at the back of
the book, along with other elements like the index and appendices. The specific
location that they are positioned can be controlled by the author using insert
directives, discussed earlier.

There is only one syntax to create a quiz or exercise. For a quiz, it's by
wrapping the quiz in `{quiz}` ... `{/quiz}`; for an exercise, it's by wrapping
the exercise in `{exercise}` ... `{/exercise}`.

Here is a brief example of a quiz:

~~~
{quiz, id: quiz1}
? How many letters are in the word Markua?

a) 5
B) 6
c) 7

? How many unique letters are in the word Markua?

! 5
{/quiz}
~~~

This quiz contains two questions: a multiple-choice question where the correct
answer is b, and a fill-in-the-blank question where the correct answer is 5.
Quizzes and exercises have the same question types, discussed later.

With a quiz, the `id` attribute is **required**. This is so the identity of a
quiz can be preserved across generations of a course.

Here is the same example, but as an exercise:

~~~
{exercise, id: exercise1}
? How many letters are in the word Markua?

a) 5
B) 6
c) 7

? How many unique letters are in the word Markua?

! 5
{/exercise}
~~~

Just like with quiz, with an exercise the `id` attribute is **required**. This
is so the identity of an exercise can be preserved across generations of a
course.



A quiz or exercise can contain any Markua content, not just questions and
answers. This is true regardless of whether the quiz or exercise is in an online
course, an ebook or on paper. Note that video and audio resources don't work so
well on paper, however.

If a quiz or exercise starts with any type of heading immediately after the
`{quiz}` or `{exercise}` line, this heading's content should be considered the
name of the quiz or exercise. This can be used in a list of quizzes or exercises
produced by the Markua Processor. Typically the heading will be a chapter
heading (`# `), but section headings (`## `) and lower headings also are
supported. (The reason for this is that quizzes are sometimes top-level things,
and other times are nested inside chapters, sections or sub-sections. Some
course authors would correctly feel that the quiz should have the appropriate
level of heading given their position in the document.

Example:

~~~
{quiz, id: quiz2}
# Markua Quiz

Watch this [video](https://www.youtube.com/watch?time_continue=1&v=VOCYL-FNbr0)
of Peter explaining Markua.

? What year was that video from?

What year? Really? Did it really take that long? What was going on???

a) 2012
b) 2013
C) 2014
d) 2015

{words: 500}
? Why do you think the first version of the Markua spec took so long?

Look at the Leanpub [website](https://leanpub.com/).

! Answers could include "bootstrapped startup", the spec evolving, etc.

That's it for this quiz, and this course!

**Thanks for taking my course!**
{/quiz}
~~~

There are four types of questions supported by Markua.

1. Multiple Choice
2. Multiple Selection
3. Fill In The Blank
4. Written

These types are not specified by a `{type}` attribute. Instead, they are
inferred from properties of the answers or from other attributes of the
question.

The exact way to create these types of questions is discussed further below.



## Quiz and exercise output (M)

A Markua Processor has many degrees of freedom in terms of whether, and how,
to output quizzes and exercises. As such, no HTML mapping is shown here.

For example, when outputting an online course, the Markua Processor can
basically do whatever it wants. It can choose to output an ebook of the course
material only, and put all quizzes and exercises only in the online version.
Or it can choose to put all the quizzes only in the online version, and include
the exercises in the ebook version as well. Or it can include the quizzes and
exercises in the ebook version, but only include the answers for the exercises
in the ebook version.

If, on the other hand, a Markua Processor is outputting a textbook, it may
choose to output the quizzes in an entirely separate ebook or print-ready PDF,
for use in physical classrooms.

If a Markua Processor does output a quiz or exercise, it needs to do so in a
medium-appropriate way. This includes outputting multiple-choice questions
without showing their answers, of course. The display of fill in the blank
and essay questions varies greatly based on whether the question is in an
online quiz or exercise, in an ebook or on paper.

Markua Processors are encouraged to be creative here.

However, there are some rules. If a given quiz or exercise is output by a
Markua Processor in an ebook or a physical book, the following things must
occur:

1. The quiz or exercise must be output as questions-only, in the place in the
   document where the quiz was defined.
2. The questions must be numbered sequentially, incrementing by 1 for each
   question.
3. The questions must be numbered starting from the number specified by the
   `start-at` attribute if specified, or 1 if no `start-at` attribute is
   specified.
4. The multiple choice options in any question must be converted into a set of
   choices which all look the same, so that the correct answer is not indicated.
   Converting all choice letters to lowercase is sufficient here.
5. The answers, if provided, must be positioned somewhere separate from the
   questions, typically at the back of the book. This position can be controlled
   by the author using insert directives, discussed earlier.



## Quiz and exercise validation (M)

### An empty quiz or exercise is not an error

A quiz or exercise which contains no questions is not an error. Instead, a if a
Markua Processor encounters a quiz or exercise with no questions it must filter
the quiz or exercise from the output, optionally providing a warning to the
author.

This lets authors create placeholders for quizzes or exercises in their courses
before the quizzes or exercises are ready, which is potentially very useful in
an in-progress course.

### A malformed quiz or exercise is an error

If a Markua Processor encounters a malformed quiz or exercise it must treat this
as an error and not generate the output from the Markua document. Quizzes and
exercises are not something that should ever be produced in a broken state.

However, it is also an error to parse quiz syntax outside a quiz or exercise
block. A Markua Processor must not parse lines starting with ? or ! as
representing questions or answers unless those are contained in a quiz or
exercise block.



## Quiz and exercise attributes (M)

These are the supported attributes on quizzes and/or exercises:

`attempts`
: The number of allowed attempts on a quiz. The default is defined by the value
of `default-quiz-attempts` on the containing course, or `1` if this is not
present. A value of `0` means the quiz cannot be taken (yet). A value of `-1`
means the quiz has an unlimited number of attempts. Since an exercise does not
count toward the mark on a course, an exercise always has an unlimited number
of attempts.

`auto-submit`
: `true` or `false`. The default is `true`. If true, an incomplete quiz is
submitted when the `time-limit` is expired. If false, it is not. Either way, an
incomplete quiz counts as an attempt.

`case-sensitive`
: `true` or `false`. The default is `true`. This sets the default behavior of
fill in the blank questions. If `true`, the fill in the blank question answers
are case-sensitive. If false, they are not.

`id`
: All Markua elements support an `id` attribute. The reason the `id` attribute
is explicitly listed here is to emphasize that a Markua Processor may require
an `id` attribute on a quiz or exercise. For example, Leanpub requires the `id`
attribute on all quizzes, in order to determine the identity of quizzes when a
course is being published in-progress. (As a student, you'd be pretty unhappy
if you had to re-take an unchanged quiz simply because a professor published a
new course version.)

`mark-request-url`
: If omitted, all the quiz or exercise answers are defined in the Markua
document. If present, some or all of the quiz or exercise questions are
externally marked via an API defined at the URL specified in this attribute.
When the quiz or exercise is completed, the entire quiz or exercise should be
sent to the endpoint at the `mark-request-url`. Here's how this works in
Leanpub; other Markua Processors should presumably do something similar. When a
quiz or exercise with this attribute defined is completed, its state is set to
"pending". Leanpub then sends the quiz or exercise data as a JSON payload to the
marking endpoint. This JSON contains the following attributes:
`mark-response-url` (which defines where responses are sent), `quiz_id`,
`quiz_version`, `quiz_hash` (an SHA hash, for an automated quiz versioning
approach which does not rely on the course author updating a version attribute),
`quiz_results` (an array of question data structures containing the question,
the possible answers, the correct answer according to the Markua document, the
answer provided by the student, and (if the question is markable) the mark
determined automatically). The API endpoint should send results to the
`mark-response-url`. (Even though exercises are not worth points in a course,
they can be marked, for the benefit of the student. However, the expectation is
that this attribute will be used primarily by quizzes.) The `mark-response-url`
in the payload contains URL to respond to with the results of the marking the
quiz or exercise. The format of the expected payload should be defined by the
Markua Processor. For Leanpub this is a JSON payload containing the question ids
and the marks for each question. The mark is a decimal number of points between
0 and the maximum number of points for the question, inclusive. The decimal
supports two decimal places, so you can get 3.75 out of 4, for example. The
mark response must include marks for all questions that are unmarked. It may
also include marks for questions that Leanpub marked, and those will override
Leanpub’s marks. Finally, note that even though exercises are not worth points
in a course, they can be marked, for the benefit of the student. So, since
they can be marked, they can also be externally-marked via a
`mark-request-url`. However, the expectation is that this attribute will be
used primarily by quizzes.

`points`
: If present, this is the total number of points the quiz or exercise is worth.
(This really only matters for quizzes, but is supported for exercises as well,
in case a Markua Processor wishes to display the points on exercises to make
them feel more real.) If `points` is not present, the worth of the quiz is
determined by summing the points of the questions. (Questions are worth 1 point
each if they have no `points` attribute.) If the quiz has a `points` attribute
and its questions also have `points` attributes, the worth of each question in
a larger course context is determined as follows: its `points` are the
numerator, and the total `points` in the quiz or exercise is the denominator.

`random-choice-order`
: `true` or `false`. The default is `false`. This sets the default behavior of
multiple choice questions. If `true`, the choices in the multiple choice
question are randomly arranged; if `false`, they are presented in the order
written.

`random-question-order`
: `true` or `false`. The default is `false`. This sets the default behavior
of the quiz or exercise. If `true`, the questions are randomly arranged; if
`false`, they are presented in the order written.

`start-at`
: The `start-at` is the number of the first question. The default is 1. Any
integer is permitted. Subsequent questions will have a number which is 1 higher
than the previous question.

`show-answers`
: This can be `all`, `incorrect` or `none`. It affects how answers are shown
after a quiz or exercise is completed, say in a course. For exercises, the
default value is determined by the value of the `default-exercise-show-answers`
document setting, with `all` being the default value of that attribute. For
quizzes, the default value is determined by the value of the
`default-quiz-show-answers` document setting, with `incorrect` being the default
value of that attribute. Document settings are discussed later.

`time-limit`
: The time limit to finish the quiz, once started. The format is `XdYhZm`. For
example, 3 days, 6 hours and 45 minutes is expressed as `3d6h45m`; 7 days is
expressed as `7d`. The default is `7d`.

`use-result`
: `best` or `latest`. Whether the best result on the quiz is used, or the latest
one. The default is the value of `default-quiz-use-result` on the course.

`version`
: The version of the quiz. This does not replace the function of the `id`; it's
more for use in analytics by the instructor. The default is `1`.

As discussed above, there is no `title` or `title` attribute for a quiz--you can
just add a heading inside the quiz or exercise itself, using the normal Markua
formatting for a chapter heading.



## Multiple choice questions (M)

A multiple choice question has 2 or more answer choices, and 1 correct answer
choice.

The correct answer choice is in capital letters before the parentheses;
incorrect answer choices have lowercase letters before the parentheses.

Example:

~~~
? How many letters are in the word Markua?

a) 5
B) 6
c) 7
~~~

Obviously, when generating the question in the actual quiz or exercise, a
Markua Processor must make all answer choices have the same type of letter.
This is usually a lowercase letter, although either all lowercase or all
uppercase letters would be fine.

Unless a `choose-answers` attribute is used, the multiple choice answers all
must start from a or A, and must use a right-parenthesis after the a or A. Any
line starting with a) ar A) in a quiz is considered a set of multiple choice
quiz answers, not an ordered list using a) or A) as a delimiter. If you want to
put an ordered list in a quiz body, use periods for the delimiter.

A multiple choice question may also have a dynamic number of answer choices,
including for the correct answer. This done with the special `choose-answers`
attribute, shown and explained below.

~~~
{choose-answers: 4}
? How many grams are in a pound?

C) 454
C) 453
m) 451
m) 1000
o) 100
o) 150
o) 200
o) 250
o) 300
o) 500
~~~

The `choose-answers` attribute specifies how many answer choices should be
shown. This includes exactly one of the correct answers (indicated with `C`),
all of the mandatory incorrect answers (indicated with `m`) and as many of the
optional incorrect answers (indicated with `o`) as are needed for the question
to have the total number of answers as indicated by the `choose-answers`
attribute.

So, in the above example, either `453` or `454` will be shown, along with the
mandatory incorrect answer choices `451` (a literary joke) and `1000` (a
kilogram, not a pound) and one of the optional incorrect answers (`100`,
`150`, `200`, `250`, `300` or `500`).

When a `choose-answers` attribute is used, the question will always have
`random-choice-order`.

The following are errors in a question where a `choose-answers` attribute is
used:
- 0 correct (`C`) answers
- not enough mandatory (`m`) incorrect or optional (`o`) incorrect answers for
the question to have the `choose-answers` number of answers
- if `choose-answers` is n, a number of mandatory (`m`) incorrect answers >= n
(since there needs to be one correct answer shown)
- if `choose-answers` is n and the number of mandatory (`m`) answers is n - 1,
then any optional (`o`) incorrect answers existing
- answers starting with something other than `C`, `m` or `o`

### Supported attributes on multiple choice questions

`choose-answers`
: This is described above. If `choose-answers` is used, `random-choice-order`
is forced to true.

`points`
: The number of points the question is worth. This number can be 0 or higher.
The default is 1.

`random-choice-order`
: `true` or `false`. The default is `false`, unless `choose-answers` is used.
This sets the behavior of the specific multiple choice question. If `true`,
the choices in the multiple choice question are randomly arranged; if `false`,
they are presented in the order written. If this attribute is omitted, its
value is determined by the `random-choice-order` attribute on the quiz itself,
which defaults to `false` if absent.



## Multiple selection questions (M)

A multiple selection question has 2 or more answer choices, and any number of
`true` answer choices.

(Leanpub authors: This question type is not yet supported in Leanpub.)

The `true` answer choices are specified with `T`; `false` answer choices are
specified with `F`.

To get the question 100% correct, you must select **ALL** answers which are
`true` (not just one of them) and **NONE** of the answers which are `false`.
As discussed later, the `mark` attribute determines whether the marking is
all-or-nothing (with the `binary` value) or whether a partial credit can be
earned (with the `ratio`) value.

Regardless of the value of the `mark` attribute, to get any score for a
multiple selection question, at least one answer choice must be selected.
Leaving a multiple selection question blank always gets zero points, even if
the `mark` attribute is `ratio`. Otherwise, if a test had nothing but multiple
selection questions with `ratio`-marked questions, with half the answers `true`
and half the answers `false`, you could get 50% on the test by just handing it
in blank!

### Supported attributes on multiple selection questions

`mark`
: `binary` or `ratio`. The default is `binary`. This sets the marking behavior
of the specific multiple selection question. If `binary`, the question is all
or nothing: you either get full points for the question, or a 0 on the question.
To get full points, you must select every `true` choice and none of the `false`
choices. If `ratio`, you get a fractional score defined as `(number of true
choices selected + number of false choices not selected) / (total number of
choices)`. This fraction is then multiplied by the `points` that the question
is worth to compute the `points` that you received on the question. Note that
regardless of whether the `mark` is `binary` or `ratio`, a question which is
left blank is always completely wrong, earning no credit for the
correctly-not-selected `false` choices.

`points`
: The number of points the question is worth. This number can be 0 or higher.
The default is 1. The `mark` attribute is used to determine how the points are
computed. Again, regardless of whether the `mark` is `binary` or `ratio`, a
question which is left blank always earns 0 points.

`random-choice-order`
: `true` or `false`. The default is `false`. This sets the behavior of the
specific multiple selection question. If `true`, the choices in the multiple
choice question are randomly arranged; if `false`, they are presented in the
order written. If this attribute is omitted, its value is determined by the
`random-choice-order` attribute on the quiz itself, which defaults to `false`
if absent.

### Examples of multiple selection questions

~~~
? Which of these are animals?

F) apple
T) cow
F) lettuce

? Which of these are animals?

F) apple
F) lettuce

? Which of these are animals?

T) cat
T) dog

{mark: ratio, points: 2}
? Which of these are fruits?

T) apple
F) cow
F) lettuce
T) peach
T) pear
F) rice
~~~

Here the first question defaults to `binary` and 1 point; the second question
is `ratio` and 2 points.

To get the first question correct, you must select `cow`, and neither of
`apple` or `lettuce`.

To get the second question correct, you must select nothing. This means that
by default someone who is skipping the question gets it right, which some
instructors may consider suboptimal. However, other instructors may consider it
amusing or philosophically interesting.

To get the third question correct, you must select everything. This is equally
controversial, but also valid.

To get this last question 100% correct, you must select all of `apple`,
`peach` and `pear`, and none of `cow`, `lettuce` or `rice`. For this question,
if the `mark` attribute had been `binary`, the only way to get any points for it
would be to select `apple`, `peach` and `pear`, and to leave `cow`, `lettuce`
and `rice` all unselected. However, the `mark` is not `binary`; it is `ratio`.
So, if you selected `apple`, `peach` and `rice`, then the score would be
computed as follows: ((2 correctly-selected true choices + 2
correctly-not-selected false choices) / 6 choices) * 2 points = (4/6) * 2
points = 1.5 points. Since at least one choice was selected, you get just as
much credit for correctly not selecting `cow` and `lettuce` as you do for
correctly selecting `apple` and `peach`. If an instructor thinks this is too
lenient, then `binary` is the choice for them!



## Fill in the blank questions (M)

A fill in the blank question consists of a question and a set of answers. Each
answer is specified by `!`, an optional points value, a space, and then a
semicolon-separated list of the acceptable values of that answer. Each answer
value can be a text string (quoted or not) or a regular expression (regex).
If a points value is not specified for an answer, the answer is worth full
points.

Support for regular expression answer values is optional. However, a Markua
Processor which supports regular expression marking must interpret any answer
which starts with a forward slash (`/`) and ends with a forward slash followed
by some word characters (e.g. `i`) as being a regular expression. Note that the
particular format of the regular expression used is implementation-specific.
For example, Leanpub uses Ruby regular expressions. Other Markua Processors
could, for example, use Perl or JavaScript regular expressions.

Finally, note that you can separate regular expressions with semicolons, just
like any other answer value. There's no reason not to support this, and it may
lead to simpler regular expressions. However, if you're good at regular
expressions, you can also combine them into one regular expression, of course.

Note that since a semicolon is used to separate answer values, to provide an
actual semicolon as part of an answer value you must either put the answer
value in quotes, use a backslash-escape `\;` or make the semicolon part of a
regular expression.

Examples:

~~~
? How many unique letters are in the word Markua?

! 5

? What's the global capital of investment banking?

! New York ; London

? What's the global capital of investment banking?

! "New York" ; "London"

? What's the global capital of investment banking?

! New York
! London

? What's the global capital of investment banking?

! "New York"
! "London"

{case-sensitive: false}
? What's pi?

! "The ratio of a circle's circumference to its diameter" ; 3.14 ... 3.1416 ;
an irrational number

{case-sensitive: false}
? What's pi?

! "The ratio of a circle's circumference to its diameter"
! 3.14 ... 3.1416
! an irrational number

? Where's the Eiffel Tower?

! /(Paris|France)/i

? Where's the Eiffel Tower?

! /Paris/i ; /France/i

{points: 2}
? Where's the Eiffel Tower?

! /Paris/i
! /France/i

{points: 2}
? Where's the Eiffel Tower?

!2 /Paris/
!1 /paris/i
!.5 /France/i

{points: 2}
? Where's the Eiffel Tower?

! /Paris/
!1 /paris/i
!.5 /France/i

{points: 2}
? Where's the global capital of investment banking?

!2 New York ; London
!1 USA ; UK
~~~

As shown by the answer (`"The ratio of a circle's circumference to its diameter"
; 3.14 ... 3.1416; an irrational number`), acceptable answer values in a fill
in the blank question can be of completely different types, and numeric answer
values can be expressed as ranges (min <= x <= max), expressed as `min ... max`.
Also, this answer shows that quotes are optional around text strings. The reason
to use quotes is for clarity, or to ensure that any semicolons used are treated
as semicolons instead of as answer choice delimiters. Semicolons inside quotes
are just semicolons and do not need to be backslash-escaped. You do, however,
need to backslash-escape a quote if you want it to be treated as a literal
quote, instead of the start or end of a string.

If there are multiple answers to a fill in the blank question and an answer key
is being output by the Markua Processor for use by human markers, the acceptable
values should be clearly distinguished from each other. The recommendation is to
use an unordered list of acceptable values, one per line, but there are no
requirements here.

If you're a programmer, you may wonder what number types and formats are
supported. Are they integers? Floating point numbers? Can you use scientific
notation? And how are the numbers formatted? Are they US or Canadian numbers
(`123,456.78`) or EU numbers (`123 456,78`)?  What if there's a number that
means something different in the US and EU--does `123,456` mean `123456` or
`123.456`?

These questions are made worse by the fact that Markua can be used to create
human-marked paper quizzes as well as automatically-marked courses. So,
specifying rules which made the syntax unambiguous for online courses with
automated marking would mean that the answers for human-marked paper quizzes
could not be specified with a tolerable amount of ambiguity. So, the format of
the answer values in a fill in the blank question is out of scope of the Markua
Spec.

### Supported attributes on fill in the blank questions

`points`
: The number of points the question is worth. This number can be 0 or higher.
The default is the 1. The answers must either not specify points (in which case
they are worth the full value of points that the question is worth), or they
must specify points between 0 and the `points` value.

`case-sensitive`
: `true` or `false`. The default is `true`. This sets the behavior of the
specific fill in the blank question. If `true`, the fill in the blank question
answer is case-sensitive. If false, it is not. In the case of multiple
acceptable answer values, this attribute applies to all of them. Note that this
only applies to text string answers, not to regular expressions. For a regular
expression to be case-insensitive, you must end it with an `i` after the
closing backtick.



## Written questions (M)

A written question corresponds to short answer, long answer or essay questions
in traditional tests. It looks the same as a fill in the blank question, except
the answer is optional.

A question is interpreted as a written question if either the `words`, `lines`
or `pages` attribute is specified, if there is no answer provided, or if an
answer is provided using the `{answer}` syntax discussed shortly.

Note that an answer may be provided in a written question. If this is done, the
answer is not split into answer choices and values like a fill in the blank
answer. Instead, the answer is essentially a "note to markers", whether those
markers are underpaid graduate students or unpaid AIs. Markua does not specify
any microformat for this note to markers: it is just Markua text, kind of like
a blockquote (but with each line starting with `!` not `>`).

A Markua Processor generating an online course may handle written questions at
its own discretion, including not including them or giving them a `points` value
of 0.

Examples:

~~~
{quiz, id: "quiz3"}

{pages: 2, points: 10}
? Why is 2019 like *1984*?

! If the student mentions Newspeak, give them an extra mark.

{words: 100, points: 2}
? Why is doubleplusungood the worst?

{lines: 10}
? Can you have multiple line answers in a written question?

! You bet you can!
!
! You can separate them with blank lines,
! and without blank lines.
!
! This is like a blockquote, which uses `>` characters.
~~~

Since a written question can have long answers, this increases the likelihood
that typing these answers after a bunch of `! ` delimiters will be a pain. So,
written questions (and **only** written questions) also support answers in the
form of `{answer} ... {/answer}`, like this:

~~~
{quiz, points: 0, id: "quiz3"}

{lines: 10}
? Write a function in Ruby that takes an argument and returns three times that
argument.

{answer}
The most straightforward way to do this is like this, since the last value
evaluated in the function is returned:

```ruby
def triple(y)
  y*3
end
```

However, you can also explicitly say `return` if you like:

```ruby
def triple(y)
  return y*3
end
```
{/answer}
{/quiz}
~~~

### Supported attributes on written questions

Note that only one of `words`, `lines` or `pages` may be provided. Providing
more than one is an error. Providing none of them means that the question is a
fill in the blank question, not a written question.

`points`
: The number of points the question is worth. This number can be 0 or higher.
The default is 1. A Markua Processor generating a course may override this. For
example, when generating a course, Leanpub overrides all written questions to
have a points attribute of 0, regardless of what (if anything) is specified for
this attribute by the author.

`words`
: The maximum number of words the answer can be. Obviously this is more useful
for online quizzes than paper ones.

`lines`
: The maximum number of lines the answer can be. Obviously this is more useful
for paper quizzes than online ones. On paper, this is most useful for a short
or long answer question.

`pages`
: The maximum number of pages the answer can be. Obviously this is more useful
for paper quizzes than online ones. On paper, this is most useful for an essay
question.



## Hints on questions (M)

Any question can contain a hint, regardless of question type or whether an
answer is provided.

The hint starts on a line with a percent sign (`%`) followed by a single space.
It must follow the question, but it can come either before or after the answer
choices.

Hints can span multiple lines, if each line starts with a `%` sign. This is
similar to Markdown handles block quotes with `>`.

The hint can be used by a Markua processor to show to students at appropriate
times, such as when they get the answer wrong or leave it blank. The exact
specifics are implementation-dependent. Hint support is optional in a Markua
Processor. However, if it's not supported, it must be ignored.

~~~
? What's 1 + 2?

% In a multiple choice question, if you're not sure, `c` is usually a good guess.

a) 1
b) 2
C) 3
d) 4

? Explain the meaning of *The Myth of Sisyphus*.

% Not the actual myth, the essay by Camus.
%
% For bonus points, speculate about Camus' life, death, and what he would have
% thought about autonomous cars.

? Where's the Eiffel Tower?

! /(Paris|France)/i

% The answer must contain the city and/or country.
~~~

The hint can be a useful feature for a Markua Processor. There is a lot of
discretion here for Markua Processors to compete on features.

For example, here's how Leanpub handles hints:

1. We will show the hint as a popup next to the question in the web and mobile
   quiz views.
2. We will track if the student looked at the hint.
3. We will not subtract points for looking at hints for Leanpub-marked questions.
4. Whether the hint was used will be provided in the analytics, such as a CSV
   download that we make available to professors.
5. We will also output the hints for exercises in the "material book" for a
   course, in a section near the end of the book. This will be linked to via
   crosslinks from each exercise.
6. The answers page for a quiz or exercise will show the question, hint and
   answers for each question.



## Question alternates (M)

The fact that a Markua document can be used to create an online course means
that certain aspects of the syntax for quizzes and exercises are more robust
than they would otherwise. One example of this is question alternates.

In an online course, some professors might not want every question the same,
despite the fact that question order and answer order can be randomized. So,
Markua supports question alternates, using a simple (if slightly ugly) syntax.
Question alternates are only supported in quizzes, since they make no sense to
include in exercises.

To create question alternates, every question in the quiz (not just those with
alternates) must be numbered sequentially, starting from 1, using a `?#` syntax.
This is a question mark followed by the number of the question, e.g.
`?1, ?2, ?3`. The questions in a quiz are numbered using sequential positive
integers starting from 1: 1, 2, 3, etc.

The alternates are specified by providing the same number for multiple
questions, e.g. `?1, ?1, ?1, ?2, ?3, ?4, ?4, ?5`. When the actual quiz is given,
only one of the questions for the given question number is used.

Note that only the first question with a given number may have a `points`
attribute--since all other alternates must use the same points value, specifying
it would be pointless.

The following is an example of a quiz which uses question alternates. This
ensures that to ensure that students get randomly selected versions of
questions 1 and 4. Also, since `random-question-order: true` is used, the actual
position of the questions is randomized after the specific questions are
selected from the alternates.

~~~
{quiz, id: "midterm", random-question-order: true}
?1 What's 2 + 2?

! 4

?1 What's 2.2 + 2.2?

! 4.4

?2 what's 3 + 3?

! 6

?3 What's 4 + 4?

! 8

{points: 2}
?4 What's 5 + 5?

! 10

?4 What's 6 + 6?

! 12

?5 What's 7 + 7?

! 14
{/quiz}
~~~

Note that the syntax for question alternates is very strict. Every question
must have a number, and these numbers must be in ascending order (except for
the alternates, which have the same number as each other).

Question alternates can also be grouped by a `choose-questions` attribute
attached to the first question alternate. In this case, the Markua Processor
must choose the number of questions `m` specified from the given alternates
with that number `n`, or `n choose m`. Note that in this scenario, the numbering
after the alternates increases by `m`: for example, if a quiz starts with a
`choose: 3`, the next question is numbered 4, not 2. This ensures that the
person constructing the quiz knows what they are doing, and saves them from
having to keep track in a scenario where there are multiple questions with a
`choose-questions` attribute.

~~~
{quiz, id: "midterm", random-question-order: true}

{choose-questions: 3}
?1 What's 2 + 2?

! 4

?1 What's 2.2 + 2.2?

! 4.4

?1 what's 3 + 3?

! 6

?1 What's 4 + 4?

! 8

?1 What's 5 + 5?

! 10

{points: 2}
?4 What's 6 + 6?

! 12

?5 What's 7 + 7?

! 14
{/quiz}
~~~

A Markua Processor must treat any error in the numbering of question
alternates (and the questions which follow) as an error, and not generate the
quiz if there is any error. This is preferable to a Markua Processor of trying
to guess at what the author meant, and trying to do the right thing. Fixing a
syntax error takes a couple minutes of editing and a few minutes to publish the
book or course again. However, fixing the consequences of a quiz being
administered to hundreds--or thousands, or tens of thousands--of people with an
incorrect number of questions, or with questions incorrectly used as alternates
for each other, would be much more difficult.

</div>



# Inlines

Inlines are parsed sequentially from the beginning of the character
stream to the end (left to right, in left-to-right languages).
Thus, for example, in

```````````````````````````````` example
`hi`lo`
.
<p><code>hi</code>lo`</p>
````````````````````````````````

`hi` is parsed as code, leaving the backtick at the end as a literal
backtick.



## Code spans

A [backtick string](@)
is a string of one or more backtick characters (`` ` ``) that is neither
preceded nor followed by a backtick.

A [code span](@) begins with a backtick string and ends with
a backtick string of equal length.  The contents of the code span are
the characters between these two backtick strings, normalized in the
following ways:

- First, [line endings] are converted to [spaces].
- If the resulting string both begins *and* ends with a [space]
  character, but does not consist entirely of [space]
  characters, a single [space] character is removed from the
  front and back.  This allows you to include code that begins
  or ends with backtick characters, which must be separated by
  whitespace from the opening or closing backtick strings.

This is a simple code span:

```````````````````````````````` example
`foo`
.
<p><code>foo</code></p>
````````````````````````````````


Here two backticks are used, because the code contains a backtick.
This example also illustrates stripping of a single leading and
trailing space:

```````````````````````````````` example
`` foo ` bar ``
.
<p><code>foo ` bar</code></p>
````````````````````````````````


This example shows the motivation for stripping leading and trailing
spaces:

```````````````````````````````` example
` `` `
.
<p><code>``</code></p>
````````````````````````````````

Note that only *one* space is stripped:

```````````````````````````````` example
`  ``  `
.
<p><code> `` </code></p>
````````````````````````````````

The stripping only happens if the space is on both
sides of the string:

```````````````````````````````` example
` a`
.
<p><code> a</code></p>
````````````````````````````````

Only [spaces], and not [unicode whitespace] in general, are
stripped in this way:

```````````````````````````````` example
` b `
.
<p><code> b </code></p>
````````````````````````````````

No stripping occurs if the code span contains only spaces:

```````````````````````````````` example
` `
`  `
.
<p><code> </code>
<code>  </code></p>
````````````````````````````````


[Line endings] are treated like spaces:

```````````````````````````````` example
``
foo
bar  
baz
``
.
<p><code>foo bar   baz</code></p>
````````````````````````````````

```````````````````````````````` example
``
foo 
``
.
<p><code>foo </code></p>
````````````````````````````````


Interior spaces are not collapsed:

```````````````````````````````` example
`foo   bar 
baz`
.
<p><code>foo   bar  baz</code></p>
````````````````````````````````

Note that browsers will typically collapse consecutive spaces
when rendering `<code>` elements, so it is recommended that
the following CSS be used:

    code{white-space: pre-wrap;}


Note that backslash escapes do not work in code spans. All backslashes
are treated literally:

```````````````````````````````` example
`foo\`bar`
.
<p><code>foo\</code>bar`</p>
````````````````````````````````


Backslash escapes are never needed, because one can always choose a
string of *n* backtick characters as delimiters, where the code does
not contain any strings of exactly *n* backtick characters.

```````````````````````````````` example
``foo`bar``
.
<p><code>foo`bar</code></p>
````````````````````````````````

```````````````````````````````` example
` foo `` bar `
.
<p><code>foo `` bar</code></p>
````````````````````````````````


Code span backticks have higher precedence than any other inline
constructs except HTML tags and autolinks.  Thus, for example, this is
not parsed as emphasized text, since the second `*` is part of a code
span:

```````````````````````````````` example
*foo`*`
.
<p>*foo<code>*</code></p>
````````````````````````````````


And this is not parsed as a link:

```````````````````````````````` example
[not a `link](/foo`)
.
<p>[not a <code>link](/foo</code>)</p>
````````````````````````````````


Code spans, HTML tags, and autolinks have the same precedence.
Thus, this is code:

```````````````````````````````` example
`<a href="`">`
.
<p><code>&lt;a href=&quot;</code>&quot;&gt;`</p>
````````````````````````````````


But this is an HTML tag:

```````````````````````````````` example
<a href="`">`
.
<p><a href="`">`</p>
````````````````````````````````


And this is code:

```````````````````````````````` example
`<https://foo.bar.`baz>`
.
<p><code>&lt;https://foo.bar.</code>baz&gt;`</p>
````````````````````````````````


But this is an autolink:

```````````````````````````````` example
<https://foo.bar.`baz>`
.
<p><a href="https://foo.bar.%60baz">https://foo.bar.`baz</a>`</p>
````````````````````````````````


When a backtick string is not closed by a matching backtick string,
we just have literal backticks:

```````````````````````````````` example
```foo``
.
<p>```foo``</p>
````````````````````````````````


```````````````````````````````` example
`foo
.
<p>`foo</p>
````````````````````````````````

The following case also illustrates the need for opening and
closing backtick strings to be equal in length:

```````````````````````````````` example
`foo``bar``
.
<p>`foo<code>bar</code></p>
````````````````````````````````


## Emphasis and strong emphasis

John Gruber's original [Markdown syntax
description](https://daringfireball.net/projects/markdown/syntax#em) says:

> Markdown treats asterisks (`*`) and underscores (`_`) as indicators of
> emphasis. Text wrapped with one `*` or `_` will be wrapped with an HTML
> `<em>` tag; double `*`'s or `_`'s will be wrapped with an HTML `<strong>`
> tag.

This is enough for most users, but these rules leave much undecided,
especially when it comes to nested emphasis.  The original
`Markdown.pl` test suite makes it clear that triple `***` and
`___` delimiters can be used for strong emphasis, and most
implementations have also allowed the following patterns:

``` markdown
***strong emph***
***strong** in emph*
***emph* in strong**
**in strong *emph***
*in emph **strong***
```

The following patterns are less widely supported, but the intent
is clear and they are useful (especially in contexts like bibliography
entries):

``` markdown
*emph *with emph* in it*
**strong **with strong** in it**
```

Many implementations have also restricted intraword emphasis to
the `*` forms, to avoid unwanted emphasis in words containing
internal underscores.  (It is best practice to put these in code
spans, but users often do not.)

``` markdown
internal emphasis: foo*bar*baz
no emphasis: foo_bar_baz
```

The rules given below capture all of these patterns, while allowing
for efficient parsing strategies that do not backtrack.

First, some definitions.  A [delimiter run](@) is either
a sequence of one or more `*` characters that is not preceded or
followed by a non-backslash-escaped `*` character, or a sequence
of one or more `_` characters that is not preceded or followed by
a non-backslash-escaped `_` character.

A [left-flanking delimiter run](@) is
a [delimiter run] that is (1) not followed by [Unicode whitespace],
and either (2a) not followed by a [Unicode punctuation character], or
(2b) followed by a [Unicode punctuation character] and
preceded by [Unicode whitespace] or a [Unicode punctuation character].
For purposes of this definition, the beginning and the end of
the line count as Unicode whitespace.

A [right-flanking delimiter run](@) is
a [delimiter run] that is (1) not preceded by [Unicode whitespace],
and either (2a) not preceded by a [Unicode punctuation character], or
(2b) preceded by a [Unicode punctuation character] and
followed by [Unicode whitespace] or a [Unicode punctuation character].
For purposes of this definition, the beginning and the end of
the line count as Unicode whitespace.

Here are some examples of delimiter runs.

  - left-flanking but not right-flanking:

    ```
    ***abc
      _abc
    **"abc"
     _"abc"
    ```

  - right-flanking but not left-flanking:

    ```
     abc***
     abc_
    "abc"**
    "abc"_
    ```

  - Both left and right-flanking:

    ```
     abc***def
    "abc"_"def"
    ```

  - Neither left nor right-flanking:

    ```
    abc *** def
    a _ b
    ```

(The idea of distinguishing left-flanking and right-flanking
delimiter runs based on the character before and the character
after comes from Roopesh Chander's
[vfmd](https://web.archive.org/web/20220608143320/http://www.vfmd.org/vfmd-spec/specification/#procedure-for-identifying-emphasis-tags).
vfmd uses the terminology "emphasis indicator string" instead of "delimiter
run," and its rules for distinguishing left- and right-flanking runs
are a bit more complex than the ones given here.)

The following rules define emphasis and strong emphasis:

1.  A single `*` character [can open emphasis](@)
    iff (if and only if) it is part of a [left-flanking delimiter run].

2.  A single `_` character [can open emphasis] iff
    it is part of a [left-flanking delimiter run]
    and either (a) not part of a [right-flanking delimiter run]
    or (b) part of a [right-flanking delimiter run]
    preceded by a [Unicode punctuation character].

3.  A single `*` character [can close emphasis](@)
    iff it is part of a [right-flanking delimiter run].

4.  A single `_` character [can close emphasis] iff
    it is part of a [right-flanking delimiter run]
    and either (a) not part of a [left-flanking delimiter run]
    or (b) part of a [left-flanking delimiter run]
    followed by a [Unicode punctuation character].

5.  A double `**` [can open strong emphasis](@)
    iff it is part of a [left-flanking delimiter run].

6.  A double `__` [can open strong emphasis] iff
    it is part of a [left-flanking delimiter run]
    and either (a) not part of a [right-flanking delimiter run]
    or (b) part of a [right-flanking delimiter run]
    preceded by a [Unicode punctuation character].

7.  A double `**` [can close strong emphasis](@)
    iff it is part of a [right-flanking delimiter run].

8.  A double `__` [can close strong emphasis] iff
    it is part of a [right-flanking delimiter run]
    and either (a) not part of a [left-flanking delimiter run]
    or (b) part of a [left-flanking delimiter run]
    followed by a [Unicode punctuation character].

9.  Emphasis begins with a delimiter that [can open emphasis] and ends
    with a delimiter that [can close emphasis], and that uses the same
    character (`_` or `*`) as the opening delimiter.  The
    opening and closing delimiters must belong to separate
    [delimiter runs].  If one of the delimiters can both
    open and close emphasis, then the sum of the lengths of the
    delimiter runs containing the opening and closing delimiters
    must not be a multiple of 3 unless both lengths are
    multiples of 3.

10. Strong emphasis begins with a delimiter that
    [can open strong emphasis] and ends with a delimiter that
    [can close strong emphasis], and that uses the same character
    (`_` or `*`) as the opening delimiter.  The
    opening and closing delimiters must belong to separate
    [delimiter runs].  If one of the delimiters can both open
    and close strong emphasis, then the sum of the lengths of
    the delimiter runs containing the opening and closing
    delimiters must not be a multiple of 3 unless both lengths
    are multiples of 3.

11. A literal `*` character cannot occur at the beginning or end of
    `*`-delimited emphasis or `**`-delimited strong emphasis, unless it
    is backslash-escaped.

12. A literal `_` character cannot occur at the beginning or end of
    `_`-delimited emphasis or `__`-delimited strong emphasis, unless it
    is backslash-escaped.

Where rules 1--12 above are compatible with multiple parsings,
the following principles resolve ambiguity:

13. The number of nestings should be minimized. Thus, for example,
    an interpretation `<strong>...</strong>` is always preferred to
    `<em><em>...</em></em>`.

14. An interpretation `<em><strong>...</strong></em>` is always
    preferred to `<strong><em>...</em></strong>`.

15. When two potential emphasis or strong emphasis spans overlap,
    so that the second begins before the first ends and ends after
    the first ends, the first takes precedence. Thus, for example,
    `*foo _bar* baz_` is parsed as `<em>foo _bar</em> baz_` rather
    than `*foo <em>bar* baz</em>`.

16. When there are two potential emphasis or strong emphasis spans
    with the same closing delimiter, the shorter one (the one that
    opens later) takes precedence. Thus, for example,
    `**foo **bar baz**` is parsed as `**foo <strong>bar baz</strong>`
    rather than `<strong>foo **bar baz</strong>`.

17. Inline code spans, links, images, and HTML tags group more tightly
    than emphasis.  So, when there is a choice between an interpretation
    that contains one of these elements and one that does not, the
    former always wins.  Thus, for example, `*[foo*](bar)` is
    parsed as `*<a href="bar">foo*</a>` rather than as
    `<em>[foo</em>](bar)`.

These rules can be illustrated through a series of examples.

Rule 1:

```````````````````````````````` example
*foo bar*
.
<p><em>foo bar</em></p>
````````````````````````````````


This is not emphasis, because the opening `*` is followed by
whitespace, and hence not part of a [left-flanking delimiter run]:

```````````````````````````````` example
a * foo bar*
.
<p>a * foo bar*</p>
````````````````````````````````


This is not emphasis, because the opening `*` is preceded
by an alphanumeric and followed by punctuation, and hence
not part of a [left-flanking delimiter run]:

```````````````````````````````` example
a*"foo"*
.
<p>a*&quot;foo&quot;*</p>
````````````````````````````````


Unicode nonbreaking spaces count as whitespace, too:

```````````````````````````````` example
* a *
.
<p>* a *</p>
````````````````````````````````

Unicode symbols count as punctuation, too:

```````````````````````````````` example
*$*alpha.

*£*bravo.

*€*charlie.
.
<p>*$*alpha.</p>
<p>*£*bravo.</p>
<p>*€*charlie.</p>
````````````````````````````````

Intraword emphasis with `*` is permitted:

```````````````````````````````` example
foo*bar*
.
<p>foo<em>bar</em></p>
````````````````````````````````


```````````````````````````````` example
5*6*78
.
<p>5<em>6</em>78</p>
````````````````````````````````


Rule 2:

```````````````````````````````` example
_foo bar_
.
<p><em>foo bar</em></p>
````````````````````````````````


This is not emphasis, because the opening `_` is followed by
whitespace:

```````````````````````````````` example
_ foo bar_
.
<p>_ foo bar_</p>
````````````````````````````````


This is not emphasis, because the opening `_` is preceded
by an alphanumeric and followed by punctuation:

```````````````````````````````` example
a_"foo"_
.
<p>a_&quot;foo&quot;_</p>
````````````````````````````````


Emphasis with `_` is not allowed inside words:

```````````````````````````````` example
foo_bar_
.
<p>foo_bar_</p>
````````````````````````````````


```````````````````````````````` example
5_6_78
.
<p>5_6_78</p>
````````````````````````````````


```````````````````````````````` example
пристаням_стремятся_
.
<p>пристаням_стремятся_</p>
````````````````````````````````


Here `_` does not generate emphasis, because the first delimiter run
is right-flanking and the second left-flanking:

```````````````````````````````` example
aa_"bb"_cc
.
<p>aa_&quot;bb&quot;_cc</p>
````````````````````````````````


This is emphasis, even though the opening delimiter is
both left- and right-flanking, because it is preceded by
punctuation:

```````````````````````````````` example
foo-_(bar)_
.
<p>foo-<em>(bar)</em></p>
````````````````````````````````


Rule 3:

This is not emphasis, because the closing delimiter does
not match the opening delimiter:

```````````````````````````````` example
_foo*
.
<p>_foo*</p>
````````````````````````````````


This is not emphasis, because the closing `*` is preceded by
whitespace:

```````````````````````````````` example
*foo bar *
.
<p>*foo bar *</p>
````````````````````````````````


A line ending also counts as whitespace:

```````````````````````````````` example
*foo bar
*
.
<p>*foo bar
*</p>
````````````````````````````````


This is not emphasis, because the second `*` is
preceded by punctuation and followed by an alphanumeric
(hence it is not part of a [right-flanking delimiter run]:

```````````````````````````````` example
*(*foo)
.
<p>*(*foo)</p>
````````````````````````````````


The point of this restriction is more easily appreciated
with this example:

```````````````````````````````` example
*(*foo*)*
.
<p><em>(<em>foo</em>)</em></p>
````````````````````````````````


Intraword emphasis with `*` is allowed:

```````````````````````````````` example
*foo*bar
.
<p><em>foo</em>bar</p>
````````````````````````````````



Rule 4:

This is not emphasis, because the closing `_` is preceded by
whitespace:

```````````````````````````````` example
_foo bar _
.
<p>_foo bar _</p>
````````````````````````````````


This is not emphasis, because the second `_` is
preceded by punctuation and followed by an alphanumeric:

```````````````````````````````` example
_(_foo)
.
<p>_(_foo)</p>
````````````````````````````````


This is emphasis within emphasis:

```````````````````````````````` example
_(_foo_)_
.
<p><em>(<em>foo</em>)</em></p>
````````````````````````````````


Intraword emphasis is disallowed for `_`:

```````````````````````````````` example
_foo_bar
.
<p>_foo_bar</p>
````````````````````````````````


```````````````````````````````` example
_пристаням_стремятся
.
<p>_пристаням_стремятся</p>
````````````````````````````````


```````````````````````````````` example
_foo_bar_baz_
.
<p><em>foo_bar_baz</em></p>
````````````````````````````````


This is emphasis, even though the closing delimiter is
both left- and right-flanking, because it is followed by
punctuation:

```````````````````````````````` example
_(bar)_.
.
<p><em>(bar)</em>.</p>
````````````````````````````````


Rule 5:

```````````````````````````````` example
**foo bar**
.
<p><strong>foo bar</strong></p>
````````````````````````````````


This is not strong emphasis, because the opening delimiter is
followed by whitespace:

```````````````````````````````` example
** foo bar**
.
<p>** foo bar**</p>
````````````````````````````````


This is not strong emphasis, because the opening `**` is preceded
by an alphanumeric and followed by punctuation, and hence
not part of a [left-flanking delimiter run]:

```````````````````````````````` example
a**"foo"**
.
<p>a**&quot;foo&quot;**</p>
````````````````````````````````


Intraword strong emphasis with `**` is permitted:

```````````````````````````````` example
foo**bar**
.
<p>foo<strong>bar</strong></p>
````````````````````````````````


Rule 6:

```````````````````````````````` example
__foo bar__
.
<p><strong>foo bar</strong></p>
````````````````````````````````


This is not strong emphasis, because the opening delimiter is
followed by whitespace:

```````````````````````````````` example
__ foo bar__
.
<p>__ foo bar__</p>
````````````````````````````````


A line ending counts as whitespace:
```````````````````````````````` example
__
foo bar__
.
<p>__
foo bar__</p>
````````````````````````````````


This is not strong emphasis, because the opening `__` is preceded
by an alphanumeric and followed by punctuation:

```````````````````````````````` example
a__"foo"__
.
<p>a__&quot;foo&quot;__</p>
````````````````````````````````


Intraword strong emphasis is forbidden with `__`:

```````````````````````````````` example
foo__bar__
.
<p>foo__bar__</p>
````````````````````````````````


```````````````````````````````` example
5__6__78
.
<p>5__6__78</p>
````````````````````````````````


```````````````````````````````` example
пристаням__стремятся__
.
<p>пристаням__стремятся__</p>
````````````````````````````````


```````````````````````````````` example
__foo, __bar__, baz__
.
<p><strong>foo, <strong>bar</strong>, baz</strong></p>
````````````````````````````````


This is strong emphasis, even though the opening delimiter is
both left- and right-flanking, because it is preceded by
punctuation:

```````````````````````````````` example
foo-__(bar)__
.
<p>foo-<strong>(bar)</strong></p>
````````````````````````````````



Rule 7:

This is not strong emphasis, because the closing delimiter is preceded
by whitespace:

```````````````````````````````` example
**foo bar **
.
<p>**foo bar **</p>
````````````````````````````````


(Nor can it be interpreted as an emphasized `*foo bar *`, because of
Rule 11.)

This is not strong emphasis, because the second `**` is
preceded by punctuation and followed by an alphanumeric:

```````````````````````````````` example
**(**foo)
.
<p>**(**foo)</p>
````````````````````````````````


The point of this restriction is more easily appreciated
with these examples:

```````````````````````````````` example
*(**foo**)*
.
<p><em>(<strong>foo</strong>)</em></p>
````````````````````````````````


```````````````````````````````` example
**Gomphocarpus (*Gomphocarpus physocarpus*, syn.
*Asclepias physocarpa*)**
.
<p><strong>Gomphocarpus (<em>Gomphocarpus physocarpus</em>, syn.
<em>Asclepias physocarpa</em>)</strong></p>
````````````````````````````````


```````````````````````````````` example
**foo "*bar*" foo**
.
<p><strong>foo &quot;<em>bar</em>&quot; foo</strong></p>
````````````````````````````````


Intraword emphasis:

```````````````````````````````` example
**foo**bar
.
<p><strong>foo</strong>bar</p>
````````````````````````````````


Rule 8:

This is not strong emphasis, because the closing delimiter is
preceded by whitespace:

```````````````````````````````` example
__foo bar __
.
<p>__foo bar __</p>
````````````````````````````````


This is not strong emphasis, because the second `__` is
preceded by punctuation and followed by an alphanumeric:

```````````````````````````````` example
__(__foo)
.
<p>__(__foo)</p>
````````````````````````````````


The point of this restriction is more easily appreciated
with this example:

```````````````````````````````` example
_(__foo__)_
.
<p><em>(<strong>foo</strong>)</em></p>
````````````````````````````````


Intraword strong emphasis is forbidden with `__`:

```````````````````````````````` example
__foo__bar
.
<p>__foo__bar</p>
````````````````````````````````


```````````````````````````````` example
__пристаням__стремятся
.
<p>__пристаням__стремятся</p>
````````````````````````````````


```````````````````````````````` example
__foo__bar__baz__
.
<p><strong>foo__bar__baz</strong></p>
````````````````````````````````


This is strong emphasis, even though the closing delimiter is
both left- and right-flanking, because it is followed by
punctuation:

```````````````````````````````` example
__(bar)__.
.
<p><strong>(bar)</strong>.</p>
````````````````````````````````


Rule 9:

Any nonempty sequence of inline elements can be the contents of an
emphasized span.

```````````````````````````````` example
*foo [bar](/url)*
.
<p><em>foo <a href="/url">bar</a></em></p>
````````````````````````````````


```````````````````````````````` example
*foo
bar*
.
<p><em>foo
bar</em></p>
````````````````````````````````


In particular, emphasis and strong emphasis can be nested
inside emphasis:

```````````````````````````````` example
_foo __bar__ baz_
.
<p><em>foo <strong>bar</strong> baz</em></p>
````````````````````````````````


```````````````````````````````` example
_foo _bar_ baz_
.
<p><em>foo <em>bar</em> baz</em></p>
````````````````````````````````


```````````````````````````````` example
__foo_ bar_
.
<p><em><em>foo</em> bar</em></p>
````````````````````````````````


```````````````````````````````` example
*foo *bar**
.
<p><em>foo <em>bar</em></em></p>
````````````````````````````````


```````````````````````````````` example
*foo **bar** baz*
.
<p><em>foo <strong>bar</strong> baz</em></p>
````````````````````````````````

```````````````````````````````` example
*foo**bar**baz*
.
<p><em>foo<strong>bar</strong>baz</em></p>
````````````````````````````````

Note that in the preceding case, the interpretation

``` markdown
<p><em>foo</em><em>bar<em></em>baz</em></p>
```


is precluded by the condition that a delimiter that
can both open and close (like the `*` after `foo`)
cannot form emphasis if the sum of the lengths of
the delimiter runs containing the opening and
closing delimiters is a multiple of 3 unless
both lengths are multiples of 3.


For the same reason, we don't get two consecutive
emphasis sections in this example:

```````````````````````````````` example
*foo**bar*
.
<p><em>foo**bar</em></p>
````````````````````````````````


The same condition ensures that the following
cases are all strong emphasis nested inside
emphasis, even when the interior whitespace is
omitted:


```````````````````````````````` example
***foo** bar*
.
<p><em><strong>foo</strong> bar</em></p>
````````````````````````````````


```````````````````````````````` example
*foo **bar***
.
<p><em>foo <strong>bar</strong></em></p>
````````````````````````````````


```````````````````````````````` example
*foo**bar***
.
<p><em>foo<strong>bar</strong></em></p>
````````````````````````````````


When the lengths of the interior closing and opening
delimiter runs are *both* multiples of 3, though,
they can match to create emphasis:

```````````````````````````````` example
foo***bar***baz
.
<p>foo<em><strong>bar</strong></em>baz</p>
````````````````````````````````

```````````````````````````````` example
foo******bar*********baz
.
<p>foo<strong><strong><strong>bar</strong></strong></strong>***baz</p>
````````````````````````````````


Indefinite levels of nesting are possible:

```````````````````````````````` example
*foo **bar *baz* bim** bop*
.
<p><em>foo <strong>bar <em>baz</em> bim</strong> bop</em></p>
````````````````````````````````


```````````````````````````````` example
*foo [*bar*](/url)*
.
<p><em>foo <a href="/url"><em>bar</em></a></em></p>
````````````````````````````````


There can be no empty emphasis or strong emphasis:

```````````````````````````````` example
** is not an empty emphasis
.
<p>** is not an empty emphasis</p>
````````````````````````````````


```````````````````````````````` example
**** is not an empty strong emphasis
.
<p>**** is not an empty strong emphasis</p>
````````````````````````````````



Rule 10:

Any nonempty sequence of inline elements can be the contents of an
strongly emphasized span.

```````````````````````````````` example
**foo [bar](/url)**
.
<p><strong>foo <a href="/url">bar</a></strong></p>
````````````````````````````````


```````````````````````````````` example
**foo
bar**
.
<p><strong>foo
bar</strong></p>
````````````````````````````````


In particular, emphasis and strong emphasis can be nested
inside strong emphasis:

```````````````````````````````` example
__foo _bar_ baz__
.
<p><strong>foo <em>bar</em> baz</strong></p>
````````````````````````````````


```````````````````````````````` example
__foo __bar__ baz__
.
<p><strong>foo <strong>bar</strong> baz</strong></p>
````````````````````````````````


```````````````````````````````` example
____foo__ bar__
.
<p><strong><strong>foo</strong> bar</strong></p>
````````````````````````````````


```````````````````````````````` example
**foo **bar****
.
<p><strong>foo <strong>bar</strong></strong></p>
````````````````````````````````


```````````````````````````````` example
**foo *bar* baz**
.
<p><strong>foo <em>bar</em> baz</strong></p>
````````````````````````````````


```````````````````````````````` example
**foo*bar*baz**
.
<p><strong>foo<em>bar</em>baz</strong></p>
````````````````````````````````


```````````````````````````````` example
***foo* bar**
.
<p><strong><em>foo</em> bar</strong></p>
````````````````````````````````


```````````````````````````````` example
**foo *bar***
.
<p><strong>foo <em>bar</em></strong></p>
````````````````````````````````


Indefinite levels of nesting are possible:

```````````````````````````````` example
**foo *bar **baz**
bim* bop**
.
<p><strong>foo <em>bar <strong>baz</strong>
bim</em> bop</strong></p>
````````````````````````````````


```````````````````````````````` example
**foo [*bar*](/url)**
.
<p><strong>foo <a href="/url"><em>bar</em></a></strong></p>
````````````````````````````````


There can be no empty emphasis or strong emphasis:

```````````````````````````````` example
__ is not an empty emphasis
.
<p>__ is not an empty emphasis</p>
````````````````````````````````


```````````````````````````````` example
____ is not an empty strong emphasis
.
<p>____ is not an empty strong emphasis</p>
````````````````````````````````



Rule 11:

```````````````````````````````` example
foo ***
.
<p>foo ***</p>
````````````````````````````````


```````````````````````````````` example
foo *\**
.
<p>foo <em>*</em></p>
````````````````````````````````


```````````````````````````````` example
foo *_*
.
<p>foo <em>_</em></p>
````````````````````````````````


```````````````````````````````` example
foo *****
.
<p>foo *****</p>
````````````````````````````````


```````````````````````````````` example
foo **\***
.
<p>foo <strong>*</strong></p>
````````````````````````````````


```````````````````````````````` example
foo **_**
.
<p>foo <strong>_</strong></p>
````````````````````````````````


Note that when delimiters do not match evenly, Rule 11 determines
that the excess literal `*` characters will appear outside of the
emphasis, rather than inside it:

```````````````````````````````` example
**foo*
.
<p>*<em>foo</em></p>
````````````````````````````````


```````````````````````````````` example
*foo**
.
<p><em>foo</em>*</p>
````````````````````````````````


```````````````````````````````` example
***foo**
.
<p>*<strong>foo</strong></p>
````````````````````````````````


```````````````````````````````` example
****foo*
.
<p>***<em>foo</em></p>
````````````````````````````````


```````````````````````````````` example
**foo***
.
<p><strong>foo</strong>*</p>
````````````````````````````````


```````````````````````````````` example
*foo****
.
<p><em>foo</em>***</p>
````````````````````````````````



Rule 12:

```````````````````````````````` example
foo ___
.
<p>foo ___</p>
````````````````````````````````


```````````````````````````````` example
foo _\__
.
<p>foo <em>_</em></p>
````````````````````````````````


```````````````````````````````` example
foo _*_
.
<p>foo <em>*</em></p>
````````````````````````````````


```````````````````````````````` example
foo _____
.
<p>foo _____</p>
````````````````````````````````


```````````````````````````````` example
foo __\___
.
<p>foo <strong>_</strong></p>
````````````````````````````````


```````````````````````````````` example
foo __*__
.
<p>foo <strong>*</strong></p>
````````````````````````````````


```````````````````````````````` example
__foo_
.
<p>_<em>foo</em></p>
````````````````````````````````


Note that when delimiters do not match evenly, Rule 12 determines
that the excess literal `_` characters will appear outside of the
emphasis, rather than inside it:

```````````````````````````````` example
_foo__
.
<p><em>foo</em>_</p>
````````````````````````````````


```````````````````````````````` example
___foo__
.
<p>_<strong>foo</strong></p>
````````````````````````````````


```````````````````````````````` example
____foo_
.
<p>___<em>foo</em></p>
````````````````````````````````


```````````````````````````````` example
__foo___
.
<p><strong>foo</strong>_</p>
````````````````````````````````


```````````````````````````````` example
_foo____
.
<p><em>foo</em>___</p>
````````````````````````````````


Rule 13 implies that if you want emphasis nested directly inside
emphasis, you must use different delimiters:

```````````````````````````````` example
**foo**
.
<p><strong>foo</strong></p>
````````````````````````````````


```````````````````````````````` example
*_foo_*
.
<p><em><em>foo</em></em></p>
````````````````````````````````


```````````````````````````````` example
__foo__
.
<p><strong>foo</strong></p>
````````````````````````````````


```````````````````````````````` example
_*foo*_
.
<p><em><em>foo</em></em></p>
````````````````````````````````


However, strong emphasis within strong emphasis is possible without
switching delimiters:

```````````````````````````````` example
****foo****
.
<p><strong><strong>foo</strong></strong></p>
````````````````````````````````


```````````````````````````````` example
____foo____
.
<p><strong><strong>foo</strong></strong></p>
````````````````````````````````



Rule 13 can be applied to arbitrarily long sequences of
delimiters:

```````````````````````````````` example
******foo******
.
<p><strong><strong><strong>foo</strong></strong></strong></p>
````````````````````````````````


Rule 14:

```````````````````````````````` example
***foo***
.
<p><em><strong>foo</strong></em></p>
````````````````````````````````


```````````````````````````````` example
_____foo_____
.
<p><em><strong><strong>foo</strong></strong></em></p>
````````````````````````````````


Rule 15:

```````````````````````````````` example
*foo _bar* baz_
.
<p><em>foo _bar</em> baz_</p>
````````````````````````````````


```````````````````````````````` example
*foo __bar *baz bim__ bam*
.
<p><em>foo <strong>bar *baz bim</strong> bam</em></p>
````````````````````````````````


Rule 16:

```````````````````````````````` example
**foo **bar baz**
.
<p>**foo <strong>bar baz</strong></p>
````````````````````````````````


```````````````````````````````` example
*foo *bar baz*
.
<p>*foo <em>bar baz</em></p>
````````````````````````````````


Rule 17:

```````````````````````````````` example
*[bar*](/url)
.
<p>*<a href="/url">bar*</a></p>
````````````````````````````````


```````````````````````````````` example
_foo [bar_](/url)
.
<p>_foo <a href="/url">bar_</a></p>
````````````````````````````````


```````````````````````````````` example
*<img src="foo" title="*"/>
.
<p>*<img src="foo" title="*"/></p>
````````````````````````````````


```````````````````````````````` example
**<a href="**">
.
<p>**<a href="**"></p>
````````````````````````````````


```````````````````````````````` example
__<a href="__">
.
<p>__<a href="__"></p>
````````````````````````````````


```````````````````````````````` example
*a `*`*
.
<p><em>a <code>*</code></em></p>
````````````````````````````````


```````````````````````````````` example
_a `_`_
.
<p><em>a <code>_</code></em></p>
````````````````````````````````


```````````````````````````````` example
**a<https://foo.bar/?q=**>
.
<p>**a<a href="https://foo.bar/?q=**">https://foo.bar/?q=**</a></p>
````````````````````````````````


```````````````````````````````` example
__a<https://foo.bar/?q=__>
.
<p>__a<a href="https://foo.bar/?q=__">https://foo.bar/?q=__</a></p>
````````````````````````````````



## Links

A link contains [link text] (the visible text), a [link destination]
(the URI that is the link destination), and optionally a [link title].
There are two basic kinds of links in Markdown.  In [inline links] the
destination and title are given immediately after the link text.  In
[reference links] the destination and title are defined elsewhere in
the document.

A [link text](@) consists of a sequence of zero or more
inline elements enclosed by square brackets (`[` and `]`).  The
following rules apply:

- Links may not contain other links, at any level of nesting. If
  multiple otherwise valid link definitions appear nested inside each
  other, the inner-most definition is used.

- Brackets are allowed in the [link text] only if (a) they
  are backslash-escaped or (b) they appear as a matched pair of brackets,
  with an open bracket `[`, a sequence of zero or more inlines, and
  a close bracket `]`.

- Backtick [code spans], [autolinks], and raw [HTML tags] bind more tightly
  than the brackets in link text.  Thus, for example,
  `` [foo`]` `` could not be a link text, since the second `]`
  is part of a code span.

- The brackets in link text bind more tightly than markers for
  [emphasis and strong emphasis]. Thus, for example, `*[foo*](url)` is a link.

A [link destination](@) consists of either

- a sequence of zero or more characters between an opening `<` and a
  closing `>` that contains no line endings or unescaped
  `<` or `>` characters, or

- a nonempty sequence of characters that does not start with `<`,
  does not include [ASCII control characters][ASCII control character]
  or [space] character, and includes parentheses only if (a) they are
  backslash-escaped or (b) they are part of a balanced pair of
  unescaped parentheses.
  (Implementations may impose limits on parentheses nesting to
  avoid performance issues, but at least three levels of nesting
  should be supported.)

A [link title](@)  consists of either

- a sequence of zero or more characters between straight double-quote
  characters (`"`), including a `"` character only if it is
  backslash-escaped, or

- a sequence of zero or more characters between straight single-quote
  characters (`'`), including a `'` character only if it is
  backslash-escaped, or

- a sequence of zero or more characters between matching parentheses
  (`(...)`), including a `(` or `)` character only if it is
  backslash-escaped.

Although [link titles] may span multiple lines, they may not contain
a [blank line].

An [inline link](@) consists of a [link text] followed immediately
by a left parenthesis `(`, an optional [link destination], an optional
[link title], and a right parenthesis `)`.
These four components may be separated by spaces, tabs, and up to one line
ending.
If both [link destination] and [link title] are present, they *must* be
separated by spaces, tabs, and up to one line ending.

The link's text consists of the inlines contained
in the [link text] (excluding the enclosing square brackets).
The link's URI consists of the link destination, excluding enclosing
`<...>` if present, with backslash-escapes in effect as described
above.  The link's title consists of the link title, excluding its
enclosing delimiters, with backslash-escapes in effect as described
above.

Here is a simple inline link:

```````````````````````````````` example
[link](/uri "title")
.
<p><a href="/uri" title="title">link</a></p>
````````````````````````````````


The title, the link text and even 
the destination may be omitted:

```````````````````````````````` example
[link](/uri)
.
<p><a href="/uri">link</a></p>
````````````````````````````````

```````````````````````````````` example
[](./target.md)
.
<p><a href="./target.md"></a></p>
````````````````````````````````


```````````````````````````````` example
[link]()
.
<p><a href="">link</a></p>
````````````````````````````````


```````````````````````````````` example
[link](<>)
.
<p><a href="">link</a></p>
````````````````````````````````


```````````````````````````````` example
[]()
.
<p><a href=""></a></p>
````````````````````````````````

The destination can only contain spaces if it is
enclosed in pointy brackets:

```````````````````````````````` example
[link](/my uri)
.
<p>[link](/my uri)</p>
````````````````````````````````

```````````````````````````````` example
[link](</my uri>)
.
<p><a href="/my%20uri">link</a></p>
````````````````````````````````

The destination cannot contain line endings,
even if enclosed in pointy brackets:

```````````````````````````````` example
[link](foo
bar)
.
<p>[link](foo
bar)</p>
````````````````````````````````

```````````````````````````````` example
[link](<foo
bar>)
.
<p>[link](<foo
bar>)</p>
````````````````````````````````

The destination can contain `)` if it is enclosed
in pointy brackets:

```````````````````````````````` example
[a](<b)c>)
.
<p><a href="b)c">a</a></p>
````````````````````````````````

Pointy brackets that enclose links must be unescaped:

```````````````````````````````` example
[link](<foo\>)
.
<p>[link](&lt;foo&gt;)</p>
````````````````````````````````

These are not links, because the opening pointy bracket
is not matched properly:

```````````````````````````````` example
[a](<b)c
[a](<b)c>
[a](<b>c)
.
<p>[a](&lt;b)c
[a](&lt;b)c&gt;
[a](<b>c)</p>
````````````````````````````````

Parentheses inside the link destination may be escaped:

```````````````````````````````` example
[link](\(foo\))
.
<p><a href="(foo)">link</a></p>
````````````````````````````````

Any number of parentheses are allowed without escaping, as long as they are
balanced:

```````````````````````````````` example
[link](foo(and(bar)))
.
<p><a href="foo(and(bar))">link</a></p>
````````````````````````````````

However, if you have unbalanced parentheses, you need to escape or use the
`<...>` form:

```````````````````````````````` example
[link](foo(and(bar))
.
<p>[link](foo(and(bar))</p>
````````````````````````````````


```````````````````````````````` example
[link](foo\(and\(bar\))
.
<p><a href="foo(and(bar)">link</a></p>
````````````````````````````````


```````````````````````````````` example
[link](<foo(and(bar)>)
.
<p><a href="foo(and(bar)">link</a></p>
````````````````````````````````


Parentheses and other symbols can also be escaped, as usual
in Markdown:

```````````````````````````````` example
[link](foo\)\:)
.
<p><a href="foo):">link</a></p>
````````````````````````````````


A link can contain fragment identifiers and queries:

```````````````````````````````` example
[link](#fragment)

[link](https://example.com#fragment)

[link](https://example.com?foo=3#frag)
.
<p><a href="#fragment">link</a></p>
<p><a href="https://example.com#fragment">link</a></p>
<p><a href="https://example.com?foo=3#frag">link</a></p>
````````````````````````````````


Note that a backslash before a non-escapable character is
just a backslash:

```````````````````````````````` example
[link](foo\bar)
.
<p><a href="foo%5Cbar">link</a></p>
````````````````````````````````


URL-escaping should be left alone inside the destination, as all
URL-escaped characters are also valid URL characters. Entity and
numerical character references in the destination will be parsed
into the corresponding Unicode code points, as usual.  These may
be optionally URL-escaped when written as HTML, but this spec
does not enforce any particular policy for rendering URLs in
HTML or other formats.  Renderers may make different decisions
about how to escape or normalize URLs in the output.

```````````````````````````````` example
[link](foo%20b&auml;)
.
<p><a href="foo%20b%C3%A4">link</a></p>
````````````````````````````````


Note that, because titles can often be parsed as destinations,
if you try to omit the destination and keep the title, you'll
get unexpected results:

```````````````````````````````` example
[link]("title")
.
<p><a href="%22title%22">link</a></p>
````````````````````````````````


Titles may be in single quotes, double quotes, or parentheses:

```````````````````````````````` example
[link](/url "title")
[link](/url 'title')
[link](/url (title))
.
<p><a href="/url" title="title">link</a>
<a href="/url" title="title">link</a>
<a href="/url" title="title">link</a></p>
````````````````````````````````


Backslash escapes and entity and numeric character references
may be used in titles:

```````````````````````````````` example
[link](/url "title \"&quot;")
.
<p><a href="/url" title="title &quot;&quot;">link</a></p>
````````````````````````````````


Titles must be separated from the link using spaces, tabs, and up to one line
ending.
Other [Unicode whitespace] like non-breaking space doesn't work.

```````````````````````````````` example
[link](/url "title")
.
<p><a href="/url%C2%A0%22title%22">link</a></p>
````````````````````````````````


Nested balanced quotes are not allowed without escaping:

```````````````````````````````` example
[link](/url "title "and" title")
.
<p>[link](/url &quot;title &quot;and&quot; title&quot;)</p>
````````````````````````````````


But it is easy to work around this by using a different quote type:

```````````````````````````````` example
[link](/url 'title "and" title')
.
<p><a href="/url" title="title &quot;and&quot; title">link</a></p>
````````````````````````````````


(Note:  `Markdown.pl` did allow double quotes inside a double-quoted
title, and its test suite included a test demonstrating this.
But it is hard to see a good rationale for the extra complexity this
brings, since there are already many ways---backslash escaping,
entity and numeric character references, or using a different
quote type for the enclosing title---to write titles containing
double quotes.  `Markdown.pl`'s handling of titles has a number
of other strange features.  For example, it allows single-quoted
titles in inline links, but not reference links.  And, in
reference links but not inline links, it allows a title to begin
with `"` and end with `)`.  `Markdown.pl` 1.0.1 even allows
titles with no closing quotation mark, though 1.0.2b8 does not.
It seems preferable to adopt a simple, rational rule that works
the same way in inline links and link reference definitions.)

Spaces, tabs, and up to one line ending is allowed around the destination and
title:

```````````````````````````````` example
[link](   /uri
  "title"  )
.
<p><a href="/uri" title="title">link</a></p>
````````````````````````````````


But it is not allowed between the link text and the
following parenthesis:

```````````````````````````````` example
[link] (/uri)
.
<p>[link] (/uri)</p>
````````````````````````````````


The link text may contain balanced brackets, but not unbalanced ones,
unless they are escaped:

```````````````````````````````` example
[link [foo [bar]]](/uri)
.
<p><a href="/uri">link [foo [bar]]</a></p>
````````````````````````````````


```````````````````````````````` example
[link] bar](/uri)
.
<p>[link] bar](/uri)</p>
````````````````````````````````


```````````````````````````````` example
[link [bar](/uri)
.
<p>[link <a href="/uri">bar</a></p>
````````````````````````````````


```````````````````````````````` example
[link \[bar](/uri)
.
<p><a href="/uri">link [bar</a></p>
````````````````````````````````


The link text may contain inline content:

```````````````````````````````` example
[link *foo **bar** `#`*](/uri)
.
<p><a href="/uri">link <em>foo <strong>bar</strong> <code>#</code></em></a></p>
````````````````````````````````


```````````````````````````````` example
[![moon](moon.jpg)](/uri)
.
<p><a href="/uri"><img src="moon.jpg" alt="moon" /></a></p>
````````````````````````````````


However, links may not contain other links, at any level of nesting.

```````````````````````````````` example
[foo [bar](/uri)](/uri)
.
<p>[foo <a href="/uri">bar</a>](/uri)</p>
````````````````````````````````


```````````````````````````````` example
[foo *[bar [baz](/uri)](/uri)*](/uri)
.
<p>[foo <em>[bar <a href="/uri">baz</a>](/uri)</em>](/uri)</p>
````````````````````````````````


```````````````````````````````` example
![[[foo](uri1)](uri2)](uri3)
.
<p><img src="uri3" alt="[foo](uri2)" /></p>
````````````````````````````````


These cases illustrate the precedence of link text grouping over
emphasis grouping:

```````````````````````````````` example
*[foo*](/uri)
.
<p>*<a href="/uri">foo*</a></p>
````````````````````````````````


```````````````````````````````` example
[foo *bar](baz*)
.
<p><a href="baz*">foo *bar</a></p>
````````````````````````````````


Note that brackets that *aren't* part of links do not take
precedence:

```````````````````````````````` example
*foo [bar* baz]
.
<p><em>foo [bar</em> baz]</p>
````````````````````````````````


These cases illustrate the precedence of HTML tags, code spans,
and autolinks over link grouping:

```````````````````````````````` example
[foo <bar attr="](baz)">
.
<p>[foo <bar attr="](baz)"></p>
````````````````````````````````


```````````````````````````````` example
[foo`](/uri)`
.
<p>[foo<code>](/uri)</code></p>
````````````````````````````````


```````````````````````````````` example
[foo<https://example.com/?search=](uri)>
.
<p>[foo<a href="https://example.com/?search=%5D(uri)">https://example.com/?search=](uri)</a></p>
````````````````````````````````


There are three kinds of [reference link](@)s:
[full](#full-reference-link), [collapsed](#collapsed-reference-link),
and [shortcut](#shortcut-reference-link).

A [full reference link](@)
consists of a [link text] immediately followed by a [link label]
that [matches] a [link reference definition] elsewhere in the document.

A [link label](@)  begins with a left bracket (`[`) and ends
with the first right bracket (`]`) that is not backslash-escaped.
Between these brackets there must be at least one character that is not a space,
tab, or line ending.
Unescaped square bracket characters are not allowed inside the
opening and closing square brackets of [link labels].  A link
label can have at most 999 characters inside the square
brackets.

One label [matches](@)
another just in case their normalized forms are equal.  To normalize a
label, strip off the opening and closing brackets,
perform the *Unicode case fold*, strip leading and trailing
spaces, tabs, and line endings, and collapse consecutive internal
spaces, tabs, and line endings to a single space.  If there are multiple
matching reference link definitions, the one that comes first in the
document is used.  (It is desirable in such cases to emit a warning.)

The link's URI and title are provided by the matching [link
reference definition].

Here is a simple example:

```````````````````````````````` example
[foo][bar]

[bar]: /url "title"
.
<p><a href="/url" title="title">foo</a></p>
````````````````````````````````


The rules for the [link text] are the same as with
[inline links].  Thus:

The link text may contain balanced brackets, but not unbalanced ones,
unless they are escaped:

```````````````````````````````` example
[link [foo [bar]]][ref]

[ref]: /uri
.
<p><a href="/uri">link [foo [bar]]</a></p>
````````````````````````````````


```````````````````````````````` example
[link \[bar][ref]

[ref]: /uri
.
<p><a href="/uri">link [bar</a></p>
````````````````````````````````


The link text may contain inline content:

```````````````````````````````` example
[link *foo **bar** `#`*][ref]

[ref]: /uri
.
<p><a href="/uri">link <em>foo <strong>bar</strong> <code>#</code></em></a></p>
````````````````````````````````


```````````````````````````````` example
[![moon](moon.jpg)][ref]

[ref]: /uri
.
<p><a href="/uri"><img src="moon.jpg" alt="moon" /></a></p>
````````````````````````````````


However, links may not contain other links, at any level of nesting.

```````````````````````````````` example
[foo [bar](/uri)][ref]

[ref]: /uri
.
<p>[foo <a href="/uri">bar</a>]<a href="/uri">ref</a></p>
````````````````````````````````


```````````````````````````````` example
[foo *bar [baz][ref]*][ref]

[ref]: /uri
.
<p>[foo <em>bar <a href="/uri">baz</a></em>]<a href="/uri">ref</a></p>
````````````````````````````````


(In the examples above, we have two [shortcut reference links]
instead of one [full reference link].)

The following cases illustrate the precedence of link text grouping over
emphasis grouping:

```````````````````````````````` example
*[foo*][ref]

[ref]: /uri
.
<p>*<a href="/uri">foo*</a></p>
````````````````````````````````


```````````````````````````````` example
[foo *bar][ref]*

[ref]: /uri
.
<p><a href="/uri">foo *bar</a>*</p>
````````````````````````````````


These cases illustrate the precedence of HTML tags, code spans,
and autolinks over link grouping:

```````````````````````````````` example
[foo <bar attr="][ref]">

[ref]: /uri
.
<p>[foo <bar attr="][ref]"></p>
````````````````````````````````


```````````````````````````````` example
[foo`][ref]`

[ref]: /uri
.
<p>[foo<code>][ref]</code></p>
````````````````````````````````


```````````````````````````````` example
[foo<https://example.com/?search=][ref]>

[ref]: /uri
.
<p>[foo<a href="https://example.com/?search=%5D%5Bref%5D">https://example.com/?search=][ref]</a></p>
````````````````````````````````


Matching is case-insensitive:

```````````````````````````````` example
[foo][BaR]

[bar]: /url "title"
.
<p><a href="/url" title="title">foo</a></p>
````````````````````````````````


Unicode case fold is used:

```````````````````````````````` example
[ẞ]

[SS]: /url
.
<p><a href="/url">ẞ</a></p>
````````````````````````````````


Consecutive internal spaces, tabs, and line endings are treated as one space for
purposes of determining matching:

```````````````````````````````` example
[Foo
  bar]: /url

[Baz][Foo bar]
.
<p><a href="/url">Baz</a></p>
````````````````````````````````


No spaces, tabs, or line endings are allowed between the [link text] and the
[link label]:

```````````````````````````````` example
[foo] [bar]

[bar]: /url "title"
.
<p>[foo] <a href="/url" title="title">bar</a></p>
````````````````````````````````


```````````````````````````````` example
[foo]
[bar]

[bar]: /url "title"
.
<p>[foo]
<a href="/url" title="title">bar</a></p>
````````````````````````````````


This is a departure from John Gruber's original Markdown syntax
description, which explicitly allows whitespace between the link
text and the link label.  It brings reference links in line with
[inline links], which (according to both original Markdown and
this spec) cannot have whitespace after the link text.  More
importantly, it prevents inadvertent capture of consecutive
[shortcut reference links]. If whitespace is allowed between the
link text and the link label, then in the following we will have
a single reference link, not two shortcut reference links, as
intended:

``` markdown
[foo]
[bar]

[foo]: /url1
[bar]: /url2
```

(Note that [shortcut reference links] were introduced by Gruber
himself in a beta version of `Markdown.pl`, but never included
in the official syntax description.  Without shortcut reference
links, it is harmless to allow space between the link text and
link label; but once shortcut references are introduced, it is
too dangerous to allow this, as it frequently leads to
unintended results.)

When there are multiple matching [link reference definitions],
the first is used:

```````````````````````````````` example
[foo]: /url1

[foo]: /url2

[bar][foo]
.
<p><a href="/url1">bar</a></p>
````````````````````````````````


Note that matching is performed on normalized strings, not parsed
inline content.  So the following does not match, even though the
labels define equivalent inline content:

```````````````````````````````` example
[bar][foo\!]

[foo!]: /url
.
<p>[bar][foo!]</p>
````````````````````````````````


[Link labels] cannot contain brackets, unless they are
backslash-escaped:

```````````````````````````````` example
[foo][ref[]

[ref[]: /uri
.
<p>[foo][ref[]</p>
<p>[ref[]: /uri</p>
````````````````````````````````


```````````````````````````````` example
[foo][ref[bar]]

[ref[bar]]: /uri
.
<p>[foo][ref[bar]]</p>
<p>[ref[bar]]: /uri</p>
````````````````````````````````


```````````````````````````````` example
[[[foo]]]

[[[foo]]]: /url
.
<p>[[[foo]]]</p>
<p>[[[foo]]]: /url</p>
````````````````````````````````


```````````````````````````````` example
[foo][ref\[]

[ref\[]: /uri
.
<p><a href="/uri">foo</a></p>
````````````````````````````````


Note that in this example `]` is not backslash-escaped:

```````````````````````````````` example
[bar\\]: /uri

[bar\\]
.
<p><a href="/uri">bar\</a></p>
````````````````````````````````


A [link label] must contain at least one character that is not a space, tab, or
line ending:

```````````````````````````````` example
[]

[]: /uri
.
<p>[]</p>
<p>[]: /uri</p>
````````````````````````````````


```````````````````````````````` example
[
 ]

[
 ]: /uri
.
<p>[
]</p>
<p>[
]: /uri</p>
````````````````````````````````


A [collapsed reference link](@)
consists of a [link label] that [matches] a
[link reference definition] elsewhere in the
document, followed by the string `[]`.
The contents of the link label are parsed as inlines,
which are used as the link's text.  The link's URI and title are
provided by the matching reference link definition.  Thus,
`[foo][]` is equivalent to `[foo][foo]`.

```````````````````````````````` example
[foo][]

[foo]: /url "title"
.
<p><a href="/url" title="title">foo</a></p>
````````````````````````````````


```````````````````````````````` example
[*foo* bar][]

[*foo* bar]: /url "title"
.
<p><a href="/url" title="title"><em>foo</em> bar</a></p>
````````````````````````````````


The link labels are case-insensitive:

```````````````````````````````` example
[Foo][]

[foo]: /url "title"
.
<p><a href="/url" title="title">Foo</a></p>
````````````````````````````````



As with full reference links, spaces, tabs, or line endings are not
allowed between the two sets of brackets:

```````````````````````````````` example
[foo] 
[]

[foo]: /url "title"
.
<p><a href="/url" title="title">foo</a>
[]</p>
````````````````````````````````


A [shortcut reference link](@)
consists of a [link label] that [matches] a
[link reference definition] elsewhere in the
document and is not followed by `[]` or a link label.
The contents of the link label are parsed as inlines,
which are used as the link's text.  The link's URI and title
are provided by the matching link reference definition.
Thus, `[foo]` is equivalent to `[foo][]`.

```````````````````````````````` example
[foo]

[foo]: /url "title"
.
<p><a href="/url" title="title">foo</a></p>
````````````````````````````````


```````````````````````````````` example
[*foo* bar]

[*foo* bar]: /url "title"
.
<p><a href="/url" title="title"><em>foo</em> bar</a></p>
````````````````````````````````


```````````````````````````````` example
[[*foo* bar]]

[*foo* bar]: /url "title"
.
<p>[<a href="/url" title="title"><em>foo</em> bar</a>]</p>
````````````````````````````````


```````````````````````````````` example
[[bar [foo]

[foo]: /url
.
<p>[[bar <a href="/url">foo</a></p>
````````````````````````````````


The link labels are case-insensitive:

```````````````````````````````` example
[Foo]

[foo]: /url "title"
.
<p><a href="/url" title="title">Foo</a></p>
````````````````````````````````


A space after the link text should be preserved:

```````````````````````````````` example
[foo] bar

[foo]: /url
.
<p><a href="/url">foo</a> bar</p>
````````````````````````````````


If you just want bracketed text, you can backslash-escape the
opening bracket to avoid links:

```````````````````````````````` example
\[foo]

[foo]: /url "title"
.
<p>[foo]</p>
````````````````````````````````


Note that this is a link, because a link label ends with the first
following closing bracket:

```````````````````````````````` example
[foo*]: /url

*[foo*]
.
<p>*<a href="/url">foo*</a></p>
````````````````````````````````


Full and compact references take precedence over shortcut
references:

```````````````````````````````` example
[foo][bar]

[foo]: /url1
[bar]: /url2
.
<p><a href="/url2">foo</a></p>
````````````````````````````````

```````````````````````````````` example
[foo][]

[foo]: /url1
.
<p><a href="/url1">foo</a></p>
````````````````````````````````

Inline links also take precedence:

```````````````````````````````` example
[foo]()

[foo]: /url1
.
<p><a href="">foo</a></p>
````````````````````````````````

```````````````````````````````` example
[foo](not a link)

[foo]: /url1
.
<p><a href="/url1">foo</a>(not a link)</p>
````````````````````````````````

In the following case `[bar][baz]` is parsed as a reference,
`[foo]` as normal text:

```````````````````````````````` example
[foo][bar][baz]

[baz]: /url
.
<p>[foo]<a href="/url">bar</a></p>
````````````````````````````````


Here, though, `[foo][bar]` is parsed as a reference, since
`[bar]` is defined:

```````````````````````````````` example
[foo][bar][baz]

[baz]: /url1
[bar]: /url2
.
<p><a href="/url2">foo</a><a href="/url1">baz</a></p>
````````````````````````````````


Here `[foo]` is not parsed as a shortcut reference, because it
is followed by a link label (even though `[bar]` is not defined):

```````````````````````````````` example
[foo][bar][baz]

[baz]: /url1
[foo]: /url2
.
<p>[foo]<a href="/url1">bar</a></p>
````````````````````````````````


<div class="extension">

## Crosslinks and ids (M)

There are two parts to making a crosslink.

1. Define an id.
2. Reference that id with a crosslink.

### Defining an id

There are two ways to define an id:

1. Using an id attribute `{id: some-id}`
2. Using a shorter "syntactic sugar" approach: `{#some-id}`

The shorter "syntactic sugar" approach is usually preferred. However, it can
look a bit odd in an attribute list with other attributes in it. So, when other
attributes are present in an attribute list, the `{id: some-id}` syntax is
preferred.

In terms of the value of an `id`, it has some special restrictions:

1. The first character in the `id` has to be a lowercase or uppercase letter,
   i.e. `[a-zA-Z]` if you think in regular expressions.
2. The remaining characters in the `id` have to be a lowercase or uppercase
   letter or a digit or a hyphen (-) or an underscore (_).
3. You can only define an `id` value once in an entire Markua document, even
   one that is split over multiple files.

These restrictions ensure that your `id`s can then be linked to by a crosslink
from anywhere in the Markua document.

Also, note that id attributes need to be defined on either a block or span
element. Finally, if an id is defined with an invalid name, the Markua Processor
must ignore it and log an error.

#### Defining an id on a block element

To define an id on a block element like a paragraph, figure, heading or even a
definition list item, you simply stick the id definition on a line above the
start of the block element. Note that exactly one newline must separate the
attribute list from the block element--if two newlines are used, the attribute
list will be interpreted as a directive, and the id won't be correctly applied.

Here's how to use the attribute list syntax to define an id attribute:

~~~
{id: some-id}
This is a paragraph with the id of `some-id`.
~~~

Here's how to use the shorter "syntactic sugar" approach to define an id
attribute:

~~~
{#some-id}
This is a paragraph with the id of `some-id`.
~~~

#### Defining an id on a span element

To define an id on a span element you simply add the id definition immediately
after the span element.

Here's how to use the attribute list syntax to define an id attribute on a span
element:

~~~
The word Markua{id: markua} has an id.

Leanpub is based in **Victoria, BC, Canada**{id: victoria}.
~~~

Here's how to use the shorter "syntactic sugar" approach to define an id
attribute:

~~~
The word Markua{#markua} has an id.

Leanpub is based in **Victoria, BC, Canada**{#victoria}.
~~~

Here's how to define an id on a custom span:

~~~
The [quick sly fox]{#quick_sly} jumped over the lazy dogs.
~~~

If you want to define an id on a span while also defining other attributes like
index entries, the `id:` syntax must be used in a full attribute list:

~~~
The [quick sly fox]{id: quick_sly, i: "Fox, Sly and Quick"} jumped over the lazy
dogs.
~~~

### Referencing an `id` With a Crosslink

Regardless of how you defined the id, you then link to it to create a crosslink.
To do this, you use the `#` character and the id in a link:

~~~
[link text](#some-id)
~~~

This syntax is intended to be reminiscent of HTML anchor tags.

Note that order of definition and use does not matter: crosslinks will work
regardless of whether the `id` is defined before or after the use of it.


```````````````````````````````` example
{id: id1}
This is a paragraph with an id of `id1`.

{#id2}
This is a paragraph with an id of `id2`.

This link to [Markua](#markua) is before the id definition.

The word Markua{id: markua} has an id of `markua`.

This link to [Markua](#markua) is after the id definition.

Leanpub is based in **Victoria, BC, Canada**{#victoria}.

The [quick sly fox]{#quick_sly} jumped over the lazy dogs.
.
<p id="id1">This is a paragraph with an id of <code>id1</code>.</p>
<p id="id2">This is a paragraph with an id of <code>id2</code>.</p>
<p>This link to <a href="#markua">Markua</a> is before the id definition.</p>
<p>The word <span id="markua">Markua</span> has an id of <code>markua</code>.</p>
<p>This link to <a href="#markua">Markua</a> is after the id definition.</p>
<p>Leanpub is based in <strong id="victoria">Victoria, BC, Canada</strong>.
<p>The <span id="quick_sly">quick sly fox</span> jumped over the lazy dogs.</p>
````````````````````````````````

### Rules for `id`s and Crosslinks

* If a Markua document contains duplicate `id` attribute values, the **first**
one is used and the subsequent ones are ignored. A Markua Processor should log
an error about duplicate `id`s.
* Crosslinks that reference an unused `id` may either be created as a (broken,
non-functional) link or be created as normal text (not a link) by a Markua
Processor. The Markua Processor should also log an error.

</div>



<div class="extension">

## Smart crosslinks (M)

Parts, chapters, sections and figures (with titles) often have two useful
properties for writers:

1. A name, which is often short and useful to reference.
2. A number.

In your writing, it's often desirable to refer to these figures from elsewhere
in your book. When creating such a reference, it's helpful to be able to
reference the title and number of the part, chapter, section or figure.

It is also helpful (especially in a print book) to be able to reference
a part, chapter, section or figure by its page number. This is a bit complex.
Whether page numbers are output in smart crosslinks is determined by the
`include-page-numbers-in-references` document setting, which defaults to
`true`. It is also determined by the output format: page numbers are supported
in PDF, but HTML and EPUB do not support outputting page numbers in references.
For EPUB, static page numbers in smart crosslinks make no sense because EPUB
files have resizable text (and thus the page numbers vary). For HTML, page
numbers make no sense, period.

### The simplest way to add a smart crosslink

The simplest way to use smart crosslinks is to always use them the same way:

~~~
Some text [#f](#its-id) more text.
~~~

The `#f` inserts the full title of what is being crosslinked to, by the
id referenced by `(#its-id)`.

If you like simplicity, stop reading this section now. If you like complexity
and fine-grained control, please continue!

### The myriad ways to add a smart crosslink

There are actually a whole bunch of different ways te insert smart crosslinks,
since you don't always want to show the full title.

Here are a number of representative examples of how to use smart crosslinks.
The actual meaning of all the references (with the `#` characters) is explained
afterward:

~~~
This is discussed in [#f](#crosslinks).

This is discussed in [section #n, #t](#crosslinks).

This is discussed in a [#d](#crosslinks) above.

See [#f](#metadata), which is the best chapter in this book.

See [chapter #n](#metadata), which is the best chapter in this book.

See [#d #n](#metadata), which is the best chapter in this book.

See [#s](#metadata), which is the best chapter in this book.

See [#f](#span-elements), which is the best section in this book.

See [section #n](#span-elements), which is the best section in this book.

This is in [#f](#fancy-diagram), arguably the fanciest diagram in this
document.

This is in [#s](#fancy-diagram), arguably the fanciest diagram in this
document.

This is in figure [#n](#fancy-diagram), arguably the fanciest diagram in this
document.
~~~

Here's how these references to titles and numbers work:

* `#t` is for "title"
* `#n` is for "number"
* `#d` is for "description" (e.g. "Figure", "Chapter", "Section")
* `#s` is for "short title" (e.g. "Figure 5", "Chapter 3", "Section 3.2")
* `#f` is for "full title"

So, for a "Figure 8.2: Anatomy of a Squirrel" on page 42, these are:

* `#t` is "Anatomy of a Squirrel"
* `#n` is "8.2"
* `#d` is "Figure"
* `#s` is "Figure 8.2"
* `#f` is "Figure 8.2: Anatomy of a Squirrel" in EPUB. In PDF, if
  `include-page-numbers-in-references` is true (the default), it is
  "Figure 8.2: Anatomy of a Squirrel on page 42". In PDF, if
  `include-page-numbers-in-references` is false, it is "Figure 8.2:
  Anatomy of a Squirrel" (just like EPUB).

Note that for a book which is in English:

* `#s` is the equivalent of the author typing `#d #n`
* `#f` is equivalent to the author typing: `#s: #t`

In other languages, the Markua Processor should do the right thing when
outputting `#s` and `#f`.

Note that in this example, "Anatomy of a Squirrel" was typed by the author,
whereas "Figure 8.2: " was generated by the Markua Processor. It does not
matter; both can be referenced.

Also, note that regardless of section level, sections referenced in `#d` or `#f`
are all called "Section" (not "Sub-Section", "Sub-Sub-Section", etc.)

The expectation is that `#f` will be used by authors who don't mind verbosity,
and `#t` and `#n` will be used by authors who prefer control and brevity.
The #d is for very lazy authors who like saving keystrokes and/or who don't know
whether their publisher will call the code samples "Listing", "Example" or some
other word and want to protect themselves against extra work.

Finally, note that you can make a smart crosslink to any id, including that of
a paragraph or span inside some chapter, section or sub-section. What the
Markua Processor should do is the following:

* `#t` is the title (descriptive name) of the most specific containing block
  (eg. "chapter", "section"). The Markua Processor is free to use "section"
  for all sections and sub-sections, regardless of depth, since
  "section 5.4.3.2.1" reads better than "sub-sub-section 5.4.3.2.1".
* `#n` is for "number", which is the number of the most specific containing
  block (eg. "3", "3.2", "5.4.3.2.1", etc.).
* `#d` is for "description" (e.g. "Figure", "Chapter", "Section") of the
  nearest containing addressable block (a block that can be referenced by name
  and/or number, i.e. "chapter", "section", "figure", "table" but not "span" or
  "paragraph")
* `#s` is for "short title", which uses the output of `#d` and `#n` in a
  language-specific way (in English, `#d #n`)
* `#f` is for "full title", which uses the output of `#d`, `#n` and `#t` in a
  language-specific way (in English, `#d #n: #t`), as well as adding the page
  number in a language-specific way (in English `#d #n: #t  on page <number>`
  [without the `<>`] if the `include-page-numbers-in-references` document
  setting is true, and `#d #n: #t` if the `include-page-numbers-in-references`
  document setting is false.

So, for a "Figure 8.2: Anatomy of a Squirrel" on page 42, these are:

* `#t` is "Anatomy of a Squirrel"
* `#n` is "8.2"
* `#d` is "Figure"
* `#s` is "Figure 8.2"
* `#f` is "Figure 8.2: Anatomy of a Squirrel" in EPUB. In PDF, if
  `include-page-numbers-in-references` is true (the default), it is
  "Figure 8.2: Anatomy of a Squirrel on page 42". In PDF, if
  `include-page-numbers-in-references` is false, it is "Figure 8.2: Anatomy
  of a Squirrel" (just like EPUB).

The `code-sample-names`, `figure-names` and `table-names` settings control the
words used to name things.

The only way to reference the title and numbers is in crosslinks. There's no
syntax to do so without creating a crosslink--a crosslink is more helpful,
since it is clickable, and adding another syntax simply to be less helpful to
the reader is a bad idea. Markua has enough syntax as it is.

Finally, note that parts, chapters and sections and figures all have implicit
numbering. So, `#n` always works even if numbering is off. However, you will
confuse readers if you refer to numbering they cannot see. Also, if numbering
is off, the `#f` must not include either the `#d` or `#n` parts: it will be
"Anatomy of a Squirrel" not "Figure 8.2: Anatomy of a Squirrel".
</div>



## Images

Syntax for images is like the syntax for links, with one
difference. Instead of [link text], we have an
[image description](@).  The rules for this are the
same as for [link text], except that (a) an
image description starts with `![` rather than `[`, and
(b) an image description may contain links.
An image description has inline elements
as its contents.  When an image is rendered to HTML,
this is standardly used as the image's `alt` attribute.

```````````````````````````````` example
![foo](/url "title")
.
<p><img src="/url" alt="foo" title="title" /></p>
````````````````````````````````


```````````````````````````````` example
![foo *bar*]

[foo *bar*]: train.jpg "train & tracks"
.
<p><img src="train.jpg" alt="foo bar" title="train &amp; tracks" /></p>
````````````````````````````````


```````````````````````````````` example
![foo ![bar](/url)](/url2)
.
<p><img src="/url2" alt="foo bar" /></p>
````````````````````````````````


```````````````````````````````` example
![foo [bar](/url)](/url2)
.
<p><img src="/url2" alt="foo bar" /></p>
````````````````````````````````


Though this spec is concerned with parsing, not rendering, it is
recommended that in rendering to HTML, only the plain string content
of the [image description] be used.  Note that in
the above example, the alt attribute's value is `foo bar`, not `foo
[bar](/url)` or `foo <a href="/url">bar</a>`.  Only the plain string
content is rendered, without formatting.

```````````````````````````````` example
![foo *bar*][]

[foo *bar*]: train.jpg "train & tracks"
.
<p><img src="train.jpg" alt="foo bar" title="train &amp; tracks" /></p>
````````````````````````````````


```````````````````````````````` example
![foo *bar*][foobar]

[FOOBAR]: train.jpg "train & tracks"
.
<p><img src="train.jpg" alt="foo bar" title="train &amp; tracks" /></p>
````````````````````````````````


```````````````````````````````` example
![foo](train.jpg)
.
<p><img src="train.jpg" alt="foo" /></p>
````````````````````````````````


```````````````````````````````` example
My ![foo bar](/path/to/train.jpg  "title"   )
.
<p>My <img src="/path/to/train.jpg" alt="foo bar" title="title" /></p>
````````````````````````````````


```````````````````````````````` example
![foo](<url>)
.
<p><img src="url" alt="foo" /></p>
````````````````````````````````


```````````````````````````````` example
![](/url)
.
<p><img src="/url" alt="" /></p>
````````````````````````````````


Reference-style:

```````````````````````````````` example
![foo][bar]

[bar]: /url
.
<p><img src="/url" alt="foo" /></p>
````````````````````````````````


```````````````````````````````` example
![foo][bar]

[BAR]: /url
.
<p><img src="/url" alt="foo" /></p>
````````````````````````````````


Collapsed:

```````````````````````````````` example
![foo][]

[foo]: /url "title"
.
<p><img src="/url" alt="foo" title="title" /></p>
````````````````````````````````


```````````````````````````````` example
![*foo* bar][]

[*foo* bar]: /url "title"
.
<p><img src="/url" alt="foo bar" title="title" /></p>
````````````````````````````````


The labels are case-insensitive:

```````````````````````````````` example
![Foo][]

[foo]: /url "title"
.
<p><img src="/url" alt="Foo" title="title" /></p>
````````````````````````````````


As with reference links, spaces, tabs, and line endings, are not allowed
between the two sets of brackets:

```````````````````````````````` example
![foo] 
[]

[foo]: /url "title"
.
<p><img src="/url" alt="foo" title="title" />
[]</p>
````````````````````````````````


Shortcut:

```````````````````````````````` example
![foo]

[foo]: /url "title"
.
<p><img src="/url" alt="foo" title="title" /></p>
````````````````````````````````


```````````````````````````````` example
![*foo* bar]

[*foo* bar]: /url "title"
.
<p><img src="/url" alt="foo bar" title="title" /></p>
````````````````````````````````


Note that link labels cannot contain unescaped brackets:

```````````````````````````````` example
![[foo]]

[[foo]]: /url "title"
.
<p>![[foo]]</p>
<p>[[foo]]: /url &quot;title&quot;</p>
````````````````````````````````


The link labels are case-insensitive:

```````````````````````````````` example
![Foo]

[foo]: /url "title"
.
<p><img src="/url" alt="Foo" title="title" /></p>
````````````````````````````````


If you just want a literal `!` followed by bracketed text, you can
backslash-escape the opening `[`:

```````````````````````````````` example
!\[foo]

[foo]: /url "title"
.
<p>![foo]</p>
````````````````````````````````


If you want a link after a literal `!`, backslash-escape the
`!`:

```````````````````````````````` example
\![foo]

[foo]: /url "title"
.
<p>!<a href="/url" title="title">foo</a></p>
````````````````````````````````



<div class="extension">

## Image resource attributes (M)

As shown in the Images section, the syntax to insert an image in Markua is
the identical to that used by CommonMark and GFM. (That section is unchanged
from the CommonMark spec.) It is, however, also consistent with the syntax that
is used for other resource types.

The following are the image resource formats and the file extensions which
choose them by default:

`jpeg`
: JPEG image - `.jpg` or `.jpeg`

`gif`
: GIF image - `.gif`

`png`
: PNG image - `.png`

`svg`
: SVG image - `.svg`

`svgz`
: SVG image (zipped) - `.svgz`

Markua uses the image syntax for all resource types. Markua also reinterprets
images as being a resource. This is important for the following reason:

Resources can have attribute lists.

Therefore, since images are resources in Markua, images can have attribute
lists. These attributes are used to do basic image formatting.

Most formatting in Markua is semantic--things like bold and italic, superscript,
etc. This formatting is part of the manuscript and should not be changed by any
publisher. The formatting for images, however, is some of the most non-semantic
formatting in Markua. It's stuff which can absolutely be changed with no effect
on the meaning of the Markua document. Specifically, images can have `width`,
`height`, `align` or `float` attributes, or can even be `fullbleed`.

These settings are so universal when applied to images that it would be too
dogmatic for Markua to insist that the only way to format images was to add a
`class` element, and then style the `class` of the image elsewhere. Markua is
pragmatic: by providing a few basic ways to format images, it enables authors
to make in-progress books look good enough to actually publish in-progress.

The following are the supported attributes for image resources, in
addition to the `class`, `format`, `title` and `type` attributes which all
resources support:

`align`
: The `align` can be `left`, `right` or `middle`. The default is `middle`.
Combined with `width` and `height`, this provides basic image formatting. When
`align` is specified, text goes above and below the image, but the image and
its surrounding whitespace occupies the entire width of the page. If you want
the text to actually wrap around the image, use `float` instead. In terms of
the specific values of `align`, a Markua Processor must interpret `left` as
"on the left side of the page", `right` as "on the right side of the page" and
`middle` as "in the middle of the content area of the page, respecting margins"
in all cases. Finally, note that `inside` and `outside` are not supported for
`align`.

`alt`
: The `alt` is the alt text, to be displayed when the image cannot be shown.
This is provided in the figure attribute list or in the square brackets before
the image, for local and web images. (Inline images don't have the square
brackets, so for an inline image, the alt text can only be provided via an
attribute.) If it is provided in both places, the one in the attribute list wins.

`float`
: The `float` can be `left`, `right`, `inside` or `outside`. The `left` and
`right` attribute values mean the same thing as they do with `align`. When
`float` is specified, text flows around the image. It is an error to specify
both `align` and `float`. Note that `middle` is not supported for `float`,
since Markua is not a good choice for the types of advanced layouts which flow
text on both sides of an image. For that, you should use something like
InDesign; this is why Markua Processors such as Leanpub can export InDesign
files. Also, note that `float` supports `inside` or `outside`, but `align`
does not. What a Markua Processor does with `inside` and `outside` is more
complex. A Markua Processor must interpret `inside` as "near the spine" and
`outside` as "away from the spine" in a print book. In an ebook, however, a
Markua Processor has some flexibility in terms of the meaning of `inside` and
`outside`: it can either interpret `inside` as "left" or it can interpret
`inside` as "left if the language is left-to-right, and right if the language
is right-to-left". Similarly, a Markua Processor can either interpret `outside`
as "right" or it can interpret `outside` as "right if the language is
left-to-right, and left if the language is right-to-left".

`fullbleed`
: `true` or `false`. The default is `false`. If `true`, the Markua Processor
should ensure that the image is the full size of the page, with no margins on
the page. If `false`, the `width`, `height` and `align` attributes are used
instead and margins are respected. A Markua Processor should do something
fancy and tasteful with the `title` attribute for a `fullbleed` image.

`height`
: The height of the image, in percentage of page content area height (respecting
margins). This is specified as a number (integer or float) between 1 and 100
followed by a percentage sign (%).

`width`
: The width of the image, in percentage of page content area width (respecting
margins). This is specified as an number (integer or float) between 1 and 100
followed by a percentage sign (%).

Note that if only one of `width` and `height` are specified, the Markua
Processor should scale the image proportionally if possible (again, respecting
margins). If both `width` and `height` are specified, the Markua Processor
should scale the image accordingly, ignoring the aspect ratio. (So, it's almost
always a bad idea to specify both `width` and `height`.)

```````````````````````````````` example
![foo](pie.jpg)

{alt: "foo"}
![](pie.jpg)

{alt: "foo"}
![clobbered by alt foo](pie.jpg)
.
<p><img src="pie.jpg" alt="foo" /></p>
<p><img src="pie.jpg" alt="foo" /></p>
<p><img src="pie.jpg" alt="foo" /></p>
````````````````````````````````
 
```````````````````````````````` example
![foo](pie.jpg "bar")

{alt: "foo"}
![](pie.jpg "bar")

{alt: "foo"}
![clobbered by alt foo](pie.jpg "bar")
.
<p><img src="pie.jpg" alt="foo" title="bar" /></p>
<p><img src="pie.jpg" alt="foo" title="bar" /></p>
<p><img src="pie.jpg" alt="foo" title="bar" /></p>
````````````````````````````````

Since the other attributes are all presentational, there is no formal
specification of the HTML which must result. So, here are some examples of usage
instead, involving a very real and very delicious apple pie:

```
{fullbleed: true}
![a half-eaten apple pie](pie.jpg "It Tasted Even Better")

{width: 100%}
![a half-eaten apple pie](pie.jpg "It Tasted Even Better")

{width: "100%", height: '30%'}
![a half-eaten apple pie](pie.jpg "It Tasted Even Better")

{width: 50%, float: left}
![a half-eaten apple pie](pie.jpg "It Tasted Even Better")

{width: 50%, align: middle}
![a half-eaten apple pie](pie.jpg "It Tasted Even Better")
```

</div>



<div class="extension">

## Inline SVG images (M)

Local and web resource locations are supported for any type of image; inline
resource locations are supported for SVG images only.

Since an SVG image is just XML text, it can be contained inline in the text of a
Markua document. This is not true for binary resources like PNG or JPEG images
or any type of audio or video file--these can only be local or web resources.

To add an inline SVG image, you create a fenced code block, and then you
indicate that instead of code, you are inserting a resource of type image
and of format SVG.

There are two ways to do this.  Here's the verbose way:

~~~
{type: image, format: svg}
```
<svg width="20" height="20">
  <circle cx="10" cy="10" r="9" fill="blue"/>
</svg>
```
~~~

Here's the syntactic sugar way:

~~~
```!
<svg width="20" height="20">
  <circle cx="10" cy="10" r="9" fill="blue"/>
</svg>
```
~~~

The `!` after the opening code fence means that what follows is an SVG image,
and that it should be rendered as an image, not displayed as the XML source of
the SVG image.

Inline SVG images support all the normal image resource attributes:

~~~
{alt: "a blue circle", title: "Earth From Space (Simplified)"}
```!
<svg width="20" height="20">
  <circle cx="10" cy="10" r="9" fill="blue"/>
</svg>
```
~~~

You can use them with the verbose way as well:

~~~
{type: image, format: svg, alt: "foo", title: "bar"}
```
<svg width="20" height="20">
  <circle cx="10" cy="10" r="9" fill="blue"/>
</svg>
```
~~~

Note that when you are writing about SVG and want to display the SVG text, what
you are really doing is creating a code resource. This is discussed below.

### Writing about SVG

If you want to write about the SVG format, and show the actual SVG source
(instead of the image produced), it needs to be of a `format` of `code`, not
`image`.

Now, you can just be lazy and not provide `format` or `type` attributes at all,
since guessing when neither is present always produces a type of `code`.

~~~
```
<svg width="20" height="20">
  <circle cx="10" cy="10" r="9" fill="blue"/>
</svg>
```
~~~

However, you can also just specify both, either this way...

~~~
{type: code}
```svg
<svg width="20" height="20">
  <circle cx="10" cy="10" r="9" fill="blue"/>
</svg>
```
~~~

...or this way:

~~~
{type: code, format: svg}
```
<svg width="20" height="20">
  <circle cx="10" cy="10" r="9" fill="blue"/>
</svg>
```
~~~

...or this way:

~~~

```svg
<svg width="20" height="20">
  <circle cx="10" cy="10" r="9" fill="blue"/>
</svg>
```
~~~

Or you can just specify xml, since SVG is xml:

~~~
```xml
<svg width="20" height="20">
  <circle cx="10" cy="10" r="9" fill="blue"/>
</svg>
```
~~~

Or you can just specify text, if you don't want syntax highlighting:

~~~
```text
<svg width="20" height="20">
  <circle cx="10" cy="10" r="9" fill="blue"/>
</svg>
```
~~~

Heck, you can even use tildes to do that:

```
~~~
<svg width="20" height="20">
  <circle cx="10" cy="10" r="9" fill="blue"/>
</svg>
~~~
```

</div>



<div class="extension">

## Image locations and embedding (M)

Note that regardless of the image location, a Markua Processor can handle images
in the following ways:

1. Embed the image in the output format (such as a PDF or EPUB file)
2. Place the image at some URL on the web, and load it from a URL in all cases

This is true regardless of the location of the source image (local, web or
inline). The location of the image source has no effect on the location of the
image in the output.

For example, a web image does not need to be loaded from the web every time; it
can be inlined in the output by the Markua Processor.

This is also true for all of the types of images which are supported in Markua:
GIF, PNG, JPEG, SVG and zipped SVG.

</div>



<div class="extension">

## Video resources (M)

Note that .mp4 is used for MP4 video, not MP4 AAC audio.

The following are the video resource formats and the file extensions which
choose them by default:

`mp4`
: MP4 video - `.mp4`

`webm`
: WebM video - `.webm`

The syntax to insert a video is the same compact and consistent syntax that is
used for any resource. Local and web resource locations are supported for both
video formats; inline resource locations for video are obviously not supported.

Unlike images where many images will accompany the manuscript itself, with
video it's expected that authors will be uploading their videos to sites such
as YouTube first, and then reusing these videos in the contents of their Markua
books. So, while video can be local or web video, web video will be much more
prominent than web images in a Markua book.

Furthermore, unlike with web images where the format (and thus the type) are
typically discoverable from the file extension in the URL, with web video it's
usually not. So, for web videos, there will typically be a `{type: video}`
attribute list at a minimum. That said, if the type and format are not
specified and the resource is a web resource, the Markua Processor may use the
domain to decide what type of resource to assume. So YouTube videos may just
work in some Markua Processors, like Leanpub, without specifying either the
`type` or `format`.

The following types of videos are supported in Markua: MP4 and WebM.

We will discuss the supported and the default attributes for videos, and then
show examples of videos being inserted for both local and web videos.

### Supported Attributes for Video

The following are the supported attributes for video resources, in addition to
the `class`, `format`, `title` and `type` attributes which all resources
support. Note that the `class`, `height`, `title` and `width` attributes apply
to the `poster` image for the video--not to the video itself during playback.

`align`
: The `align` can be `left`, `right` or `middle`. This applies to the `poster`
image for the video, and works just like it does with images.

`embed`
: true or false. If true, then you can actually embed the video file or
reference it and play it. If false, then it's from a site like YouTube which
disallows this. In this case, this should function like a link to external web
page, but open an appropriate app (e.g. the YouTube app) instead of a browser.
A Markua Processor can be smart about defaults, and parse the URL to set the
value of the `embed` attribute.

`float`
: The `float` can be `left`, `right`, `inside` or `outside`.  This applies to
the `poster` image for the video, and works just like it does with images. It is
an error to specify both `align` and `float`.

`height`
: This applies to the `poster` image for the video, and works just like it does
with images.

`poster`
: The `poster` is the URL or path to an image which should be shown instead of
the video before the video is played. If a Markua Processor is outputting some
format where it is known that video resources are not supported, it must choose
the poster to use as a replacement for the video. Books are not just
ebooks--books can also be printed on the fibers of trees that have been chopped
down ("paper"), producing something called a "book". These "books", whether
they are bound in a sturdy fashion ("hardcover books") or a flimsy fashion
("paperback books"), have one thing in common with respect to embedded video:
they do not support it. Finally, if a Markua Processor is embedding a web video
from a web video service (e.g. YouTube) which has a known algorithm for the
location of the poster images for its videos, the Markua Processor may choose
to use that poster image if a `poster` attribute is not explicitly specified.
However, if a poster attribute is specified, then it must be used by the Markua
Processor.

`poster-format`
: The format of the `poster` image, if it exists. If this attribute is not
specified, the format of the `poster` is inferred from the `poster` image file
extension. This attribute needs to exist only to support `poster` images which
do not have file extensions.

`transcript`
: The URL or path to a transcript of the video, which should be shown or
dictated to people who cannot view video. This helps people with visual
disabilities view course material, and helps with ADA compliance. For example,
what Leanpub does with a `transcript` is as follows: a URL is linked to in the
title (but in a way which does not affect the Table of Contents), and a path is
assumed to be a path to a Markua file which is then used to produce a web page
at a public URL, which then is also linked to in the title in a way which does
not affect the Table of Contents. With the Markua files for transcripts, Leanpub
only supports certain resource types to be included in the transcript files
themselves. Specifically, images, video, audio are not supported, but code,
math and tables are supported. When Leanpub generates a transcript, the URLs are
publicly accessible but obscure. Identical transcripts should not make duplicate
transcript files on the web, and using a new UUID every time would violate this.
However, not changing the transcript URL when its content changed could lead to
bad outcomes. So, the URL of a transcript on the web should be scoped to the
book and affected both by its filename and its content. What Leanpub does is:
`(filename minus extension minus period plus hyphen plus the SHA hash).html`
Including the filename eliminates collisions if filenames are unique but the
hashes are not.

`width`
: This applies to the `poster` image for the video, and works just like it does
with images.

In the following sections, please note that while the examples are shown with
an HTML mapping, please note that a Markua Processor has complete flexibility
over how it handles the location of video resources and their display.

### Local Video

```````````````````````````````` example
Here's a paragraph before the figure.

![](pie_eating_contest.webm "A Piece of Pie")

Here's a paragraph after the figure.
.
<p>Here's a paragraph before the figure.</p>
<div class="figure">
<video src="pie_eating_contest.webm"/>
<p class="title">A Piece of Pie</p>
</div>
<p>Here's a paragraph after the figure.</p>
````````````````````````````````

### Web Video

```````````````````````````````` example
Here's a paragraph before the figure.

{type: video, poster: https://img.youtube.com/vi/VOCYL-FNbr0/mqdefault.jpg}
![](https://www.youtube.com/watch?t=105&v=VOCYL-FNbr0 "Introducing Markua")

Here's a paragraph after the figure.
.
<p>Here's a paragraph before the figure.</p>
<div class="figure">
<video src="https://www.youtube.com/watch?t=105&v=VOCYL-FNbr0"
poster="https://img.youtube.com/vi/VOCYL-FNbr0/mqdefault.jpg"/>
<p class="title">Introducing Markua</p>
</div>
<p>Here's a paragraph after the figure.</p>
````````````````````````````````

</div>



<div class="extension">

## IFrame resources (M)

The syntax to insert an IFrame is almost identical to that is used for video
resources.

The idea about having a syntax to insert an IFrame is simple: since Markua
does not support inline HTML, there is no implicit support for it. However,
resources such as slide decks can be an important part of, say, courses which
are created in Markua.

So, the resource syntax is used for yet another task.

The only resource location which is supported for an IFrame resource is the
web resource location, for obvious reasons.

### Supported Attributes for `iframe` Resources

The following are the supported attributes for IFrame resources, in addition to
the `class`, `format`, `title` and `type` attributes which all resources
support.

`align`
: The `align` can be `left`, `right` or `middle`.

`height`
: This works just like it does with images.

`poster`
: The `poster` is the URL or path to an image which should be shown instead of
the IFrame. If a Markua Processor is outputting some format where it is known
that IFrame resources are not supported, it must choose the poster to use as
a replacement for the IFrame. This poster is then turned into a link to a
web page displaying the IFrame content, either top-level or as an IFrame.

`poster-format`
: The format of the `poster` image, if it exists. If this attribute is not
specified, the format of the `poster` is inferred from the `poster` image file
extension. This attribute needs to exist only to support `poster` images which
do not have file extensions.

`transcript`
: The URL or path to a transcript of the resource contained in the IFrame,
which should be shown or dictated to people who cannot view the IFrame content.
This helps people with visual disabilities view course material, and helps with
ADA compliance. For example, what Leanpub does with an `iframe` resource
`transcript` is the same as what it does for a `video` transcript.

`width`
: This works just like it does with images.

In the following sections, please note that while the examples are shown with
an HTML mapping, please note that a Markua Processor has complete flexibility
over how it handles the location of video resources and their display.

```````````````````````````````` example
Here's a paragraph before the figure.

{type: iframe, poster: https://markua.com/Markua.png, width: 300, height: 200}
![](https://markua.com/Markua.pdf "Introducing Markua")

Here's a paragraph after the figure.
.
<p>Here's a paragraph before the figure.</p>
<div class="figure">
<iframe title="Introducing Markua"
    width="300"
    height="200"
    src="https://markua.com/Markua.pdf">
<p class="title">Introducing Markua</p>
</iframe>
</div>
<p>Here's a paragraph after the figure.</p>
````````````````````````````````

</div>



<div class="extension">

## Audio resources (M)

The following are the audio resource formats and the file extensions which
choose them by default:

`aac`
: AAC  audio - `.aac`, `.m4a`

`mp3`
: MP3 audio - `.mp3`

`ogg`
: Ogg Vorbis audio - `.oga`, `.ogg`

`wav`
: WAV audio - `.wav`, `.wave`

The syntax to insert an audio resource is the same compact and consistent syntax
that is used for any resource. Local and web resource locations are supported
for both audio formats; inline resource locations for audio are obviously not
supported.

Just as with video, the audio support in ebooks and on the web is more varied
than for images. With audio, there are MP3, AAC, Ogg and WAV formats all in
widespread use, and there are a number of other formats with supporters. It's
entirely likely that many ebook readers won't support any of them.

Unlike images where many images will accompany the manuscript itself, with
audio it's expected that authors will be uploading their audio files to various
sites first, and then reusing these files in the contents of their Markua
books. So, while audio can be local or web audio, web audio will be much more
prominent than web images in a Markua book.

Furthermore, unlike with web images where the format (and thus the type) are
typically discoverable from the file extension in the URL, with web audio it's
usually not. So, for web audio files, there will typically be a `{type: audio}`
attribute list at a minimum. That said, if the type and format are not
specified and the resource is a web resource, the Markua Processor may use the
domain to decide what type of resource to assume. So YouTube audios may just
work in some Markua Processors, like Leanpub, without specifying either the
`type` or `format`.

The following types of audio resources are supported in Markua: MP3, AAC, WAV
and Ogg Vorbis.

We will discuss the supported and the default attributes for audio files, and
then show examples of audio being inserted for both local and web audio files.

### Supported Attributes for Audio

The following are the supported attributes for audio resources, in addition to
the `class`, `format`, `title` and `type` attributes which all resources
support.

`poster`
: The `poster` is the URL or path to an image (such as an album cover or a
photo of the artist) which should be shown to accompany the audio resource.
A Markua Processor may choose to make this poster be a link to the resource,
as well as making its title be a link to the resource.

`transcript`
: The URL or path to a transcript of the audio, which should be shown to people
who cannot hear audio. This helps people with auditory disabilities view course
material, and helps with ADA compliance. Transcripts should be produced and
handled in an identical way to `video` resources. For example, Leanpub does
this. See the Video resources section above for more information.

In the following sections, please note that while the examples are shown with
an HTML mapping, please note that a Markua Processor has complete flexibility
over how it handles the location of video resources and their display.


### Local Audio

```````````````````````````````` example
The full version of the talk is here:

![](talk.mp3 "Full Talk")
.
<p>The full version of the talk is here:</p>
<div class="figure">
<audio src="resources/talk.mp3"/>
<p class="title">Full Talk</p>
</div>
````````````````````````````````

### Web Audio

```````````````````````````````` example
The full version of the talk is here:

![](https://markua.com/talk.mp3 "Full Talk")
.
<p>The full version of the talk is here:</p>
<div class="figure">
<audio src="https://markua.com/talk.mp3"/>
<p class="title">Full Talk</p>
</div>
````````````````````````````````

</div>



<div class="extension">

## Math resources (M)

Math can be a local, web or inline resource, just like any other resource, and
the same resource syntax applies to code as to all other resources.

Markua does not specify how math is output in HTML. It can be an SVG or PNG
image, with or without transparency, or it can use something like MathJax.
After much research, Leanpub actually settled on using PNGs with a white
background, for maximum compatibility with browsers, older devices and dark
mode. However, a different Markua Processor could maake a different decision,
like transparent SVGs with a dynamic foreground color, or MathJax or MathML.

There are two formats of math resources supported by Markua:

1. LaTeX math (`latexmath` or `$`)
2. AsciiMath (`asciimath` or `@`)

### Math resource formats

The following are the math resource formats and the file extensions which
choose them by default:

`asciimath`
: AsciiMath math - `.asciimath`

`latexmath`
: LaTeX math - `.tex`

Note that AsciiMath and LaTeX math are almost always contained as inline
resources in a Markua document, not in external files as local or web resources.

Also, note that `.tex` is assumed to be LaTeX math, not a vanilla LaTeX file.
There is a simple reason for this: Markua needs LaTeX math for math, but only
needs to display LaTeX for people writing about LaTeX. So, if you have a LaTeX
file with an extension of `.tex` that just contains LaTeX code and you want to
display it, then add a `{type: code, format: latex}` attribute list.

Note that the assumption is that AsciiMath will almost always be used as an
inline resource. So, the `.asciimath` file extension is deliberately verbose.


### Supported Attributes for Math

The following are the supported attribute for math resources, in addition to the
`class`, `format`, `title` and `type` attributes which all resources support:

`alt`
: The `alt` is the alt text, to be displayed when the mathematical equations
cannot be shown. The default alt text for math is "math". This can be provided
in the figure attribute list. This is primarily intended for Markua Processors
that output math as images; there are no output requirements for the alt text.
This attribute functions as it does for images. (In fact, a Markua Processor may
choose to transform the math into an image, for maximum ebook reader
compatibility.)

Note that for math, the `format` is the name of the syntax used to write the
mathematical equations. There are two special types of `format` for math baked
into Markua: `latexmath` for LaTeX math and `asciimath` for AsciiMath. If a
Markua Processor encounters one of these formats, it must assume the `type` of
the resource is `math`, not `code`.


### Using bold and strikethrough formatting with math

Markua supports formatting math as bold or strikethrough.

Math can be `**bolded**` by being wrapped with two asterisks, and can be
formatted in `~~strikethrough~~` by being wrapped with two tildes. Note that
the format of the strikethrough line is controlled by the
`strikethrough-format` document setting.


### Local math resources

Local math resources can be inserted as a figure.

~~~
Here's a paragraph before the figure.

{format: latexmath}
![too large to fit in the alt text](fermat-proof.tex "Proof of Fermat's Last Theorem")

Here's a paragraph after the figure.
~~~

The alt text can also be set with an attribute list:

~~~
Here's a paragraph before the figure.

{format: latexmath, alt: "too large to fit in the alt text"}
![](fermat-proof.tex "Proof of Fermat's Last Theorem")

Here's a paragraph after the figure.
~~~

### Web math resources

This is identical to how local math resources work, including the significance
of file extensions. The only difference is that the files are on the web.

~~~
Here's a paragraph before the figure.

{format: latexmath}
![too large to fit in the alt text](https://markua.com/fermat-proof.tex "Proof of Fermat's Last Theorem")

Here's a paragraph after the figure.
~~~

The alt text can also be set with an attribute list:

~~~
Here's a paragraph before the figure.

{format: latexmath, alt: "too large to fit in the alt text"}
![](https://markua.com/fermat-proof.tex "Proof of Fermat's Last Theorem")

Here's a paragraph after the figure.
~~~

### Inline math resources

Inline math resources are the most flexible way to insert math. They are the only
way to insert math as a span resource, and the most straightforward way to add
short math examples as figures. LaTeX math and AsciiMath can be inserted inline
as a span or figure.

#### Span

Being able to insert a math resource as a span is important, as it lets you write
things like one of the kinematic equations `d = v_i t + \frac{1}{2} a t^2`$ inside
sentences. This can be done with LaTeX math or AsciiMath.

To insert math as inline math, use a `$` after closing backtick for LaTeX math,
an `@` after the closing backtick for AsciiMath, or an attribute list specifying
a `format` of `latexmath` or `asciimath`. If none of these is done, the content
of the backticks is treated as code and is output verbatim as monospaced text.

##### LaTeX math span

There is syntactic sugar for LaTeX math which is inserted as a span, using the
`$` character after the closing backtick:

~~~
Here's one of the kinematic equations `d = v_i t + \frac{1}{2} a t^2`$ inside a
sentence.
~~~

The `$` character indicates the inline resource is LaTeX math.

If you don't like syntactic sugar, you can also use `{format: latexmath}` or
`{type: math, format: latexmath}` after the inline span resource:

~~~
Here's one of the kinematic equations
`d = v_i t + \frac{1}{2} a t^2`{format: latexmath}
inside a sentence.
~~~

##### AsciiMath span

[AsciiMath](https://asciimath.org/) is a way of producing simple MathML equations,
using about 1% of the typing. It's more terse than LaTeX math.

There is syntactic sugar for AsciiMath which is inserted as a span, using the `@`
character after the closing backtick:

~~~
Here's one of the kinematic equations `d = v_i t + 1/2 at^2`@ inside a sentence.
~~~

The `@` character indicates the inline resource is AsciiMath.

If you don't like syntactic sugar, you can also use `{format: asciimath}` or
`{type: math, format: asciimath}` after the inline span resource:

~~~
Here's one of the kinematic equations `d = v_i t + 1/2 at^2`{format: asciimath}
inside a sentence.
~~~

#### Figure

LaTeX math and AsciiMath can be inserted inline as a figure.

* LaTeX math can be inserted by specifying either `latexmath` or `$` after three
  backticks, or by specifying an attribute list of `{format: latexmath}`.
* AsciiMath can be inserted by specifying either `asciimath` or `@` after three
  backticks, or by specifying an attribute list of `{format: asciimath}`.

##### LaTeX math figures

Here's how you do this using LaTeX math...

Here's the version with the syntactic sugar for the format after the backticks:

~~~
Here's a paragraph before the figure.

```$
\left|\sum_{i=1}^n a_ib_i\right|
\le
\left(\sum_{i=1}^n a_i^2\right)^{1/2}
\left(\sum_{i=1}^n b_i^2\right)^{1/2}
```

Here's a paragraph after the figure.
~~~

Here's the same thing, with the full format after the backticks:

~~~
Here's a paragraph before the figure.

```latexmath
\left|\sum_{i=1}^n a_ib_i\right|
\le
\left(\sum_{i=1}^n a_i^2\right)^{1/2}
\left(\sum_{i=1}^n b_i^2\right)^{1/2}
```

Here's a paragraph after the figure.
~~~

Here's the same thing again, with a full attribute list:

~~~
Here's a paragraph before the figure.

{format: latexmath}
```
\left|\sum_{i=1}^n a_ib_i\right|
\le
\left(\sum_{i=1}^n a_i^2\right)^{1/2}
\left(\sum_{i=1}^n b_i^2\right)^{1/2}
```

Here's a paragraph after the figure.
~~~

##### AsciiMath figures

Here's how you do this using AsciiMath...

Here's the version with the syntactic sugar for the format after the backticks:

~~~
Here's a paragraph before the figure.

```@
abs(sum_(i=1)^n a_i b_i) <= (sum_(i=1)^n a_i^2)^(1/2) (sum_(i=1)^n b_i^2)^(1/2)
```

Here's a paragraph after the figure.
~~~

Here's the same thing, with the full format after the backticks:

~~~
Here's a paragraph before the figure.

```asciimath
abs(sum_(i=1)^n a_i b_i) <= (sum_(i=1)^n a_i^2)^(1/2) (sum_(i=1)^n b_i^2)^(1/2)
```

Here's a paragraph after the figure.
~~~

Here's the same thing again, with a full attribute list:

~~~
Here's a paragraph before the figure.

{format: asciimath}
```
abs(sum_(i=1)^n a_i b_i) <= (sum_(i=1)^n a_i^2)^(1/2) (sum_(i=1)^n b_i^2)^(1/2)
```

Here's a paragraph after the figure.
~~~

If you wonder why I'm a fan of AsciiMath: I actually got that right on the first
try at the [AsciiMath website](https://asciimath.org/).

Note that when you are writing about AsciiMath and want to display the AsciiMath
text, what you are really doing is creating a code resource. This is essentially
identical to what was shown earlier in writing about SVG images. It's also shown
below.

### Writing about Math

If you want to write about math, and show the actual code (instead of the
formatted output), it needs to be of a `format` of `code`, not `math`.

Now, you can just be lazy and not provide `format` or `type` attributes at all,
since guessing when neither is present always produces a type of `code`.

This will be output as code:

~~~
```
abs(sum_(i=1)^n a_i b_i) <= (sum_(i=1)^n a_i^2)^(1/2) (sum_(i=1)^n b_i^2)^(1/2)
```
~~~

This isn't just true for AsciiMath code; it's true for any math. For example,
this will be output as code, since there's no `$` or `latexmath` format specifier
to indicate that it is LaTeX math:

~~~
```
\left|\sum_{i=1}^n a_ib_i\right|
\le
\left(\sum_{i=1}^n a_i^2\right)^{1/2}
\left(\sum_{i=1}^n b_i^2\right)^{1/2}
```
~~~

However, you can also specify the type of `code`, to be explicit:

~~~
{type: code}
```
abs(sum_(i=1)^n a_i b_i) <= (sum_(i=1)^n a_i^2)^(1/2) (sum_(i=1)^n b_i^2)^(1/2)
```
~~~

Since specifying a type overrides the type inferred by the format, you can even
specify the format of the math being used, while still keeping it code:

~~~
{type: code, format: asciimath}
```
abs(sum_(i=1)^n a_i b_i) <= (sum_(i=1)^n a_i^2)^(1/2) (sum_(i=1)^n b_i^2)^(1/2)
```
~~~

This also works with the syntactic sugar with the full format name:

~~~
{type: code}
```asciimath
abs(sum_(i=1)^n a_i b_i) <= (sum_(i=1)^n a_i^2)^(1/2) (sum_(i=1)^n b_i^2)^(1/2)
```
~~~

This also works with the syntactic sugar with the format shortcut:

~~~
{type: code}
```@
abs(sum_(i=1)^n a_i b_i) <= (sum_(i=1)^n a_i^2)^(1/2) (sum_(i=1)^n b_i^2)^(1/2)
```
~~~

Now, chances are a Markua Processor will not be doing syntax highlighting on
AsciiMath.

If you want to ensure that no syntax highlighting is done, you can just specify
`text`:

~~~
```text
abs(sum_(i=1)^n a_i b_i) <= (sum_(i=1)^n a_i^2)^(1/2) (sum_(i=1)^n b_i^2)^(1/2)
```
~~~

Heck, you can even use tildes, since this defaults to `text` not `guess`:

```
~~~
abs(sum_(i=1)^n a_i b_i) <= (sum_(i=1)^n a_i^2)^(1/2) (sum_(i=1)^n b_i^2)^(1/2)
~~~
```

</div>



## Autolinks

[Autolink](@)s are absolute URIs and email addresses inside
`<` and `>`. They are parsed as links, with the URL or email address
as the link label.

A [URI autolink](@) consists of `<`, followed by an
[absolute URI] followed by `>`.  It is parsed as
a link to the URI, with the URI as the link's label.

An [absolute URI](@),
for these purposes, consists of a [scheme] followed by a colon (`:`)
followed by zero or more characters other than [ASCII control
characters][ASCII control character], [space], `<`, and `>`.
If the URI includes these characters, they must be percent-encoded
(e.g. `%20` for a space).

For purposes of this spec, a [scheme](@) is any sequence
of 2--32 characters beginning with an ASCII letter and followed
by any combination of ASCII letters, digits, or the symbols plus
("+"), period ("."), or hyphen ("-").

Here are some valid autolinks:

```````````````````````````````` example
<http://foo.bar.baz>
.
<p><a href="http://foo.bar.baz">http://foo.bar.baz</a></p>
````````````````````````````````


```````````````````````````````` example
<https://foo.bar.baz/test?q=hello&id=22&boolean>
.
<p><a href="https://foo.bar.baz/test?q=hello&amp;id=22&amp;boolean">https://foo.bar.baz/test?q=hello&amp;id=22&amp;boolean</a></p>
````````````````````````````````


```````````````````````````````` example
<irc://foo.bar:2233/baz>
.
<p><a href="irc://foo.bar:2233/baz">irc://foo.bar:2233/baz</a></p>
````````````````````````````````


Uppercase is also fine:

```````````````````````````````` example
<MAILTO:FOO@BAR.BAZ>
.
<p><a href="MAILTO:FOO@BAR.BAZ">MAILTO:FOO@BAR.BAZ</a></p>
````````````````````````````````


Note that many strings that count as [absolute URIs] for
purposes of this spec are not valid URIs, because their
schemes are not registered or because of other problems
with their syntax:

```````````````````````````````` example
<a+b+c:d>
.
<p><a href="a+b+c:d">a+b+c:d</a></p>
````````````````````````````````


```````````````````````````````` example
<made-up-scheme://foo,bar>
.
<p><a href="made-up-scheme://foo,bar">made-up-scheme://foo,bar</a></p>
````````````````````````````````


```````````````````````````````` example
<https://../>
.
<p><a href="https://../">https://../</a></p>
````````````````````````````````


```````````````````````````````` example
<localhost:5001/foo>
.
<p><a href="localhost:5001/foo">localhost:5001/foo</a></p>
````````````````````````````````


Spaces are not allowed in autolinks:

```````````````````````````````` example
<https://foo.bar/baz bim>
.
<p>&lt;https://foo.bar/baz bim&gt;</p>
````````````````````````````````


Backslash-escapes do not work inside autolinks:

```````````````````````````````` example
<https://example.com/\[\>
.
<p><a href="https://example.com/%5C%5B%5C">https://example.com/\[\</a></p>
````````````````````````````````


An [email autolink](@)
consists of `<`, followed by an [email address],
followed by `>`.  The link's label is the email address,
and the URL is `mailto:` followed by the email address.

An [email address](@),
for these purposes, is anything that matches
the [non-normative regex from the HTML5
spec](https://html.spec.whatwg.org/multipage/forms.html#e-mail-state-(type=email)):

    /^[a-zA-Z0-9.!#$%&'*+/=?^_`{|}~-]+@[a-zA-Z0-9](?:[a-zA-Z0-9-]{0,61}[a-zA-Z0-9])?
    (?:\.[a-zA-Z0-9](?:[a-zA-Z0-9-]{0,61}[a-zA-Z0-9])?)*$/

Examples of email autolinks:

```````````````````````````````` example
<foo@bar.example.com>
.
<p><a href="mailto:foo@bar.example.com">foo@bar.example.com</a></p>
````````````````````````````````


```````````````````````````````` example
<foo+special@Bar.baz-bar0.com>
.
<p><a href="mailto:foo+special@Bar.baz-bar0.com">foo+special@Bar.baz-bar0.com</a></p>
````````````````````````````````


Backslash-escapes do not work inside email autolinks:

```````````````````````````````` example
<foo\+@bar.example.com>
.
<p>&lt;foo+@bar.example.com&gt;</p>
````````````````````````````````


These are not autolinks:

```````````````````````````````` example
<>
.
<p>&lt;&gt;</p>
````````````````````````````````


```````````````````````````````` example
< https://foo.bar >
.
<p>&lt; https://foo.bar &gt;</p>
````````````````````````````````


```````````````````````````````` example
<m:abc>
.
<p>&lt;m:abc&gt;</p>
````````````````````````````````


```````````````````````````````` example
<foo.bar.baz>
.
<p>&lt;foo.bar.baz&gt;</p>
````````````````````````````````


```````````````````````````````` example
https://example.com
.
<p>https://example.com</p>
````````````````````````````````


```````````````````````````````` example
foo@bar.example.com
.
<p>foo@bar.example.com</p>
````````````````````````````````


<div class="extension">

## No raw HTML (M)

As [discussed](#markua-s-one-key-subtraction-from-markdown-raw-html-is-removed-m-),
Markua does not support raw HTML. **All raw HTML is removed**. As a side-effect,
this means HTML comments can still be used, since they (like all HTML) will be
removed from the output.

</div>



## Hard line breaks

A line ending (not in a code span or HTML tag) that is preceded
by two or more spaces and does not occur at the end of a block
is parsed as a [hard line break](@) (rendered
in HTML as a `<br />` tag):

```````````````````````````````` example
foo  
baz
.
<p>foo<br />
baz</p>
````````````````````````````````


For a more visible alternative, a backslash before the
[line ending] may be used instead of two or more spaces:

```````````````````````````````` example
foo\
baz
.
<p>foo<br />
baz</p>
````````````````````````````````


More than two spaces can be used:

```````````````````````````````` example
foo       
baz
.
<p>foo<br />
baz</p>
````````````````````````````````


Leading spaces at the beginning of the next line are ignored:

```````````````````````````````` example
foo  
     bar
.
<p>foo<br />
bar</p>
````````````````````````````````


```````````````````````````````` example
foo\
     bar
.
<p>foo<br />
bar</p>
````````````````````````````````


Hard line breaks can occur inside emphasis, links, and other constructs
that allow inline content:

```````````````````````````````` example
*foo  
bar*
.
<p><em>foo<br />
bar</em></p>
````````````````````````````````


```````````````````````````````` example
*foo\
bar*
.
<p><em>foo<br />
bar</em></p>
````````````````````````````````


Hard line breaks do not occur inside code spans

```````````````````````````````` example
`code  
span`
.
<p><code>code   span</code></p>
````````````````````````````````


```````````````````````````````` example
`code\
span`
.
<p><code>code\ span</code></p>
````````````````````````````````


or HTML tags:

```````````````````````````````` example
<a href="foo  
bar">
.
<p><a href="foo  
bar"></p>
````````````````````````````````


```````````````````````````````` example
<a href="foo\
bar">
.
<p><a href="foo\
bar"></p>
````````````````````````````````


Hard line breaks are for separating inline content within a block.
Neither syntax for hard line breaks works at the end of a paragraph or
other block element:

```````````````````````````````` example
foo\
.
<p>foo\</p>
````````````````````````````````


```````````````````````````````` example
foo  
.
<p>foo</p>
````````````````````````````````


```````````````````````````````` example
### foo\
.
<h3>foo\</h3>
````````````````````````````````


```````````````````````````````` example
### foo  
.
<h3>foo</h3>
````````````````````````````````


## Soft line breaks

A regular line ending (not in a code span or HTML tag) that is not
preceded by two or more spaces or a backslash is parsed as a
[softbreak](@).  (A soft line break may be rendered in HTML either as a
[line ending] or as a space. The result will be the same in
browsers. In the examples here, a [line ending] will be used.)

```````````````````````````````` example
foo
baz
.
<p>foo
baz</p>
````````````````````````````````


Spaces at the end of the line and beginning of the next line are
removed:

```````````````````````````````` example
foo 
 baz
.
<p>foo
baz</p>
````````````````````````````````


A conforming parser may render a soft line break in HTML either as a
line ending or as a space.

A renderer may also provide an option to render soft line breaks
as hard line breaks.



<div class="extension">

## Character substitutions (M)

Markua documents can be written in UTF-8, so to produce any Unicode character, it
is possible to just use the proper Unicode characters. However, in certain cases,
it's desirable for Markua to specify automatic replacement of certain
combinations of characters with a Unicode replacement. If a Markua Processor
encounters one of these combinations of characters outside of a code block, the
Markua Processor must replace the combination of characters with the appropriate
Unicode character in the output.

`--`
: To produce an em dash (—), what is thought of by non-typography people as a
"dash" or a "long dash", you can just type two hyphens (`--`) directly after a
non-space character. You can also use the proper Unicode character, U+2014, of
course. The following all produce em dashes: `foo--bar, foo-- bar, foo--`.

` --`
: To produce a space followed by an en dash (–), or the kind of dash that's wider
than a hyphen but narrower than an em dash, you can just type a space, followed
by two hyphens (` --`). You can also use the proper Unicode character, U+2013,
of course. The following both produce en dashes preceded by spaces:
`foo -- bar`, `foo --`. (With `foo -- bar`, there's a space before and after the
en dash; with `foo --`, there's no space after it (e.g. at the end of a
paragraph).

`...`
: To produce a horizontal ellipsis (…), you can just type `...`. You can also use
the proper Unicode character, U+2026, of course.

### Optional Automatic Curly Quotes Outside of Code Blocks and Spans

A Markua Processor may replace the `"` character with the appropriate "curly
quote" at its discretion. This lets `"typography"` become `“typography”`, and
`it's` become `it’s` as appropriate.

Note that this is an optional behavior: a Markua Processor may support this
fully, only in some output formats, or not at all.

Also, note that it is **NEVER** acceptable for a Markua Processor to do this,
or *any* character substitution, to text inside a code block or code span. In
almost all instances this would completely break the code. (If you wonder how I
got curly quotes into the code spans for `“typography”` and `it’s` above, it's
because I pasted them into the manuscript that way. Just as a Markua Processor
doesn't make straight quotes curly in a code span, it doesn't make curly quotes
straight in a code span either.)

</div>



<div class="extension">

## Footnotes and endnotes (M)

Books often have footnotes and endnotes. So, Markua has them too.

### Footnotes

To add a footnote, you insert a footnote tag using square brackets, a caret and
the tag, like this:

~~~
This has a footnote[^thenote].
~~~

Then, you define the footnote later in the document, using the same square
brackets, caret and tag, followed by a colon, a space and the footnote
definition:

~~~
[^thenote]: This is the footnote content.
~~~

If you wish to write multiple paragraphs in the footnote, you must indent the
subsequent paragraphs by four spaces or one tab:

~~~
This has a footnote[^thenote].

Here is some unrelated text.

[^thenote]: This is the first paragraph of footnote content.

    This is the second paragraph of footnote content.

Here is some more unrelated text.
~~~

Whether the numbering of footnotes restarts every chapter is something that can
be specified by the `restart-footnote-numbering` setting.

Markua does not specify how footnotes are output in HTML. A Markua Processor
should output them *somewhere*, but the details are not specified. This is
deliberate, in order to maximize implementation flexibility for Markua
Processors.

### Endnotes

Sometimes endnotes are used instead of footnotes, but other times, these are in
addition to footnotes. So, it makes sense for Markua to define separate syntaxes
for both, rather than just defining one "footnote or endnote" syntax and letting
the author pick whether the notes are footnotes or endnotes via a document
setting.

To add an endnote, you insert an endnote tag using square brackets, two carets
and the tag, like this:

~~~
This has an endnote[^^thenote].
~~~

Endnotes are like footnotes, but happier (`^^`).

Then, you define the endnote later in the document, using the same square
brackets, two carets and tag, followed by a colon, a space and the endnote
definition:

~~~
[^^thenote]: This is the endnote content.
~~~

Just as with footnotes, if you wish to write multiple paragraphs in an endnote,
you must indent the subsequent paragraphs by four spaces or one tab.

Whether the numbering of endnotes restarts every chapter is something that can
be specified by the `restart-endnote-numbering` setting.

Markua does not specify how endnotes are output in HTML. A Markua Processor
should output them *somewhere*, but the details are not specified. This is
deliberate, in order to maximize implementation flexibility for Markua
Processors.

### Single reference to footnotes and endnotes

You can only refer to a footnote or endnote once. You can't define a footnote or
endnote in one place and refer to it multiple times in the same Markua document.
If you wish to refer to a parenthetical piece of text from multiple places in a
Markua document, the best approach is to put it in a section (or sub-section,
sub-sub-section, etc.) or aside and refer to it from multiple places using a
[crosslink](#crosslinks).

### Footnote and endnote support is required for paragraphs only

A Markua Processor must support footnote and endnote references inserted in
normal paragraph content. However, that's it.

However, sometimes authors want to get creative with their footnotes and
endnotes. Sometimes they want to add them in headings, or in footnotes or
endnotes themselves. This latter style has been used on rare occasions, most
notably by David Foster Wallace.

However, supporting inserting footnotes and endnotes in places other than normal
paragraph content puts a hugely increased burden on implementors of Markua
Processors. As such, there is no requirement for a Markua Processor to support
inserting a footnote or endnote anywhere other than in normal paragraph content.

Authors should **not** assume that a particular Markua Processor supports
inserting a footnote or endnote anywhere other than in normal paragraph content
unless its documentation specifically states that it does. For example, Leanpub
only supports inserting footnotes or endnotes in normal paragraph content.

</div>



<div class="extension">

## Underline (M)

In Markdown as defined by John Gruber, and in CommonMark and GFM,
`*one asterisk*` and `_one underscore_` both produce *italics*, and there is no
way to produce an underline in Markdown except using inline HTML.

This is unfortunate, since underline is not always just a typewriter version of
italics. In some languages and in some contexts, underlining serves a distinct,
legitimate purpose.

Worse, Markua bans all inline HTML except HTML comments. So even the gross
Markdown workaround to produce underlines with HTML does not work.

So, how to produce an actual underline?

In Markua, `*one asterisk*` produces italics, and `_one underscore_` can
produce either italics or an underline based on the `italicize-underlines`
document setting.

The document setting `italicize-underlines` can be `true` or `false`. The default
is `true`, so that Markua functions the same way as Markdown by default.

This will be a bit surprising for new authors who are discovering Markua and
have never heard of Markdown, such as people writing novels in Markua. However,
it is my expectation that for the foreseeable future, the proportion of authors
who discover Markdown first and then learn about Markua will be far greater.
For these people, it would be a lot more surprising if all their text which they
had italicized like `_this_` suddenly became underlined instead!

Furthermore, if you've written in a certain way (like `_this_` for italic) for
years, your fingers essentially just do the right thing. I want this to still
feel right for these people, not something that makes them think they need to go
change a document setting. Almost nobody has spent years writing in Markua, so
this is much less of an issue for them!

(Also, frankly, most novels just need italic, not underline, so this isn't a big
issue. While underline does have legitimate uses, they are more niche than
italic.)

So, by default, `italicize-underlines` is `true`:

```````````````````````````````` example
# Chapter One

stuff

foo _bar_ baz

lorem *ipsum* dolor

stuff
.
<h1>Chapter One</h1>
<p>stuff</p>
<p>foo <em>bar</em> baz</p>
<p>lorem <em>ipsum</em> dolor</p>
<p>stuff</p>
````````````````````````````````

This can be made explicit by setting the `italicize-underlines` document setting
to `true`:

```````````````````````````````` example
{
italicize-underlines: true
}

# Chapter One

stuff

foo _bar_ baz

lorem *ipsum* dolor

stuff
.
<h1>Chapter One</h1>
<p>stuff</p>
<p>foo <em>bar</em> baz</p>
<p>lorem <em>ipsum</em> dolor</p>
<p>stuff</p>
````````````````````````````````

Set the `italicize-underlines` document setting to `false` to produce underline:

```````````````````````````````` example
{
italicize-underlines: false
}

# Chapter One

stuff

foo _bar_ baz

lorem *ipsum* dolor

stuff
.
<h1>Chapter One</h1>
<p>stuff</p>
<p>foo <u>bar</u> baz</p>
<p>lorem <em>ipsum</em> dolor</p>
<p>stuff</p>
````````````````````````````````

To produce text which is underlined as well as other bolded and/or italicized, you combine the underscores for underlining with the asterisks for bold and italics:

```````````````````````````````` example
{
italicize-underlines: false
}

# Chapter One

stuff

This is _*italicized and underlined*_.

This is _**bolded and underlined**_.

This is _***bolded and italicized and underlined***_.

stuff
.
<h1>Chapter One</h1>
<p>stuff</p>
<p>This is <u><em>italicized and underlined</em></u>.</p>
<p>This is <u><strong>bolded and underlined</strong></u>.</p>
<p>This is <u><strong><em>bolded and italicized and underlined</em></strong></u>.</p>
<p>stuff</p>
````````````````````````````````

</div>



<div class="extension">

## Superscript and subscript (M)

To produce superscript like the 3 in 5^3^ = 125, surround it with carets like `5^3^ = 125`.

```````````````````````````````` example
Superscript: 5^3^ = 125
.
<p>Superscript: 5<sup>3</sup> = 125</p>
````````````````````````````````

To produce subscript like the 2 in H~2~O, surround it with single tildes like `H~2~O`.

```````````````````````````````` example
Subscript: H~2~O
.
<p>Subscript: H<sub>2</sub>O</p>
````````````````````````````````

</div>



<div class="extension">

## Strikethrough (GFM)

Markua enables the `strikethrough` extension, where an additional emphasis type is
available.

Strikethrough text is any text wrapped in two tildes (`~`).

```````````````````````````````` example
~~Hi~~ Hello, world!
.
<p><del>Hi</del> Hello, world!</p>
````````````````````````````````

As with regular emphasis delimiters, a new paragraph will cause strikethrough
parsing to cease:

```````````````````````````````` example
This ~~has a

new paragraph~~.
.
<p>This ~~has a</p>
<p>new paragraph~~.</p>
````````````````````````````````

</div>



<div class="extension">

## Index entries (M)

Markua supports adding index entries via the attribute list syntax. Index
entries let authors or indexers produce professional-looking indexes in
Markua books.

Index entries can either be attached to block or span elements using the same
attribute list syntax. In fact, index entries can just be added as part of a
larger attribute list.

The actual syntax of what the value of an index entry looks like is [inspired
by LaTeX](https://en.wikibooks.org/wiki/LaTeX/Indexing).

The key of an index entry is `i`, for index.

In its simplest form, an index entry is simply `{i: "hello"}`. Like any
attribute list, you don't need a space between the colon and the quote--you can
also do `{i:"hello"}`.

These are the various formats of an index entry:

~~~
{i: hello}
{i: "hello"}
{i: "Armstrong, Peter"}
{i: "Yahoo\!"}
{i: "*hello*"}
{i: "**hello**"}
{i: "hello!Peter"}
{i: "hello!*Peter*"}
{i: "hello!**Peter**"}
{i: "Peter|see{i:'hello'}"}
{i: "Jen|seealso{i:'Jenny'}"}
~~~

Here's what they do:

`{i: hello}`
: Adds an index entry for `hello`. If an index entry has no punctuation or
formatting then it does not need quotes.

`{i: "hello"}`
: Adds an index entry for `hello`. Quotes are always fine to use, even when not
required.

`{i: "Armstrong, Peter"}`
: Adds an index entry for `Armstrong, Peter`. The quotes are always omitted.
Their function is to allow things like exclamation marks and other punctuation
to be added without fear, in case you don't feel like learning which punctuation
is safe.

`{i: "Yahoo\!"}`
: Adds an index entry for `Yahoo!`. Note that the exclamation mark must be
backslash-escaped because `!` is a delimiter otherwise. The `|`, `{`, `}` and
`\` characters also must be backslash-escaped.

`{i: "*hello*"}`
: Adds an index entry for `hello`, with `hello` in italics.

`{i: "**hello**"}`
: Adds an index entry for `hello`, with `hello` in bold.

`{i: "hello!Peter"}`
: Adds an index entry for `Peter` which is a sub-entry of `hello`.

`{i: "hello!*Peter*"}`
: Adds an index entry for `Peter` (with *Peter* in emphasis) which is a
sub-entry of `hello`. Note that this cannot be combined with a see or
seealso (the | syntax).

`{i: "hello!**Peter**"}`
: Adds an index entry for `Peter` (with **Peter** in strong emphasis) which is a
sub-entry of `hello`. Note that this cannot be combined with a see or seealso
(the | syntax).

`{i: "Peter|see{i:'hello'}"}`
: Adds an index entry for `Peter`, which references the index entry `hello` with
the equivalent of "see" in the language of the book. Note that this cannot be
combined with a sub-entry (the ! syntax).

`{i: "Jen|seealso{i:'Jenny'}"}`
: Adds an index entry for `Jen`, which references the index entry `Jenny` with
the equivalent of "see also" in the language of the book. Note that this cannot
be combined with a sub-entry (the ! syntax).

Index entries are case sensitive. For example, `{i: "mark"}` and `{i: "Mark"}`
are distinct entries. (The first is for a result or a smudge, the second is a
person's name.)

To attach an index entry to the start of a block, put it on its own line above
a block:

~~~
{i: "hello"}
I just came to say hello, hello, hello, hello
~~~

To attach an index entry to a word, just add the index entry after the word:

~~~
I just came to say hello{i: "hello"}, hello, hello, hello
~~~

To attach an index entry to a span element, just add the index entry after the
span element:

~~~
The first program that a programmer writes in a language is usually
*Hello World*{i: "Hello World"}
~~~

Index entries can have commas and other punctuation (except colons) in their
definition:

~~~
My wife read some book about a whale by Herman Melville{i: "Melville, Herman"}.
~~~

Multiple index entries can exist in a block, or even a sentence:

~~~
Supposedly the great-great-great-granduncle of the musician Moby{i: "Moby"} was
Herman Melville{i: "Melville, Herman"}, the author of a book about a
whale{i: "Moby-Dick; or, The Whale"}.
~~~

Note that adding index entries is best left until the author is done writing the
book. At that time, ids like {#myid} can be converted to {id: #myid, i: "blah"}
if index entries are being added where ids already are.

Markua does not specify the HTML output of index entries, to maximize
implementation flexibility.

Finally, please note that if you use an exclamation mark (!) symbol in an
index entry, you must backslash-escape it. Otherwise, it would be intepreted
as an attempt at creating a sub-entry. (This actually got me, when I tried to
reference *Hello! Flex 4*, one of my books. To do this, I would have to write
the entry as `{i: "*Hello\! Flex 4*}` not as `{i: "*Hello! Flex 4*}`.

</div>



<div class="extension">

## Syntactic sugar list (M)

Markua has a handful of syntactic sugar that it relies on to make life nicer for
authors. Here's a helpful summary list of the different syntactic sugar elements
provided:

`$`
: When used following three backticks or tildes which start an inline resource,
adds `{type: math, format: latexmath}` to the resource.

`@`
: When used following three backticks or tildes which start an inline resource,
adds `{type: math, format: asciimath}` to the resource.

`!`
: When used following three backticks or tildes which start an inline resource,
adds `{type: image, format: svg}` to the resource.

`&`
: When used following three backticks or tildes which start an inline resource,
adds `{type: verbatim}` to the resource.

`^`
: When used as the first character on a line by itself above a paragraph, adds
a `{class: continued-para}`

three backticks to start a resource
: `{format: guess}`

three tildes to start a resource
: `{format: text}`
</div>



## Textual content

Any characters not given an interpretation by the above rules will
be parsed as plain textual content.

```````````````````````````````` example
hello $.;'there
.
<p>hello $.;'there</p>
````````````````````````````````


```````````````````````````````` example
Foo χρῆν
.
<p>Foo χρῆν</p>
````````````````````````````````


Internal spaces are preserved verbatim:

```````````````````````````````` example
Multiple     spaces
.
<p>Multiple     spaces</p>
````````````````````````````````



<!-- END TESTS -->

# Appendix: A parsing strategy

In this appendix we describe some features of the parsing strategy
used in the CommonMark reference implementations.

## Overview

Parsing has two phases:

1. In the first phase, lines of input are consumed and the block
structure of the document---its division into paragraphs, block quotes,
list items, and so on---is constructed.  Text is assigned to these
blocks but not parsed. Link reference definitions are parsed and a
map of links is constructed.

2. In the second phase, the raw text contents of paragraphs and headings
are parsed into sequences of Markdown inline elements (strings,
code spans, links, emphasis, and so on), using the map of link
references constructed in phase 1.

At each point in processing, the document is represented as a tree of
**blocks**.  The root of the tree is a `document` block.  The `document`
may have any number of other blocks as **children**.  These children
may, in turn, have other blocks as children.  The last child of a block
is normally considered **open**, meaning that subsequent lines of input
can alter its contents.  (Blocks that are not open are **closed**.)
Here, for example, is a possible document tree, with the open blocks
marked by arrows:

``` tree
-> document
  -> block_quote
       paragraph
         "Lorem ipsum dolor\nsit amet."
    -> list (type=bullet tight=true bullet_char=-)
         list_item
           paragraph
             "Qui *quodsi iracundia*"
      -> list_item
        -> paragraph
             "aliquando id"
```

## Phase 1: block structure

Each line that is processed has an effect on this tree.  The line is
analyzed and, depending on its contents, the document may be altered
in one or more of the following ways:

1. One or more open blocks may be closed.
2. One or more new blocks may be created as children of the
   last open block.
3. Text may be added to the last (deepest) open block remaining
   on the tree.

Once a line has been incorporated into the tree in this way,
it can be discarded, so input can be read in a stream.

For each line, we follow this procedure:

1. First we iterate through the open blocks, starting with the
root document, and descending through last children down to the last
open block.  Each block imposes a condition that the line must satisfy
if the block is to remain open.  For example, a block quote requires a
`>` character.  A paragraph requires a non-blank line.
In this phase we may match all or just some of the open
blocks.  But we cannot close unmatched blocks yet, because we may have a
[lazy continuation line].

2.  Next, after consuming the continuation markers for existing
blocks, we look for new block starts (e.g. `>` for a block quote).
If we encounter a new block start, we close any blocks unmatched
in step 1 before creating the new block as a child of the last
matched container block.

3.  Finally, we look at the remainder of the line (after block
markers like `>`, list markers, and indentation have been consumed).
This is text that can be incorporated into the last open
block (a paragraph, code block, heading, or raw HTML).

Setext headings are formed when we see a line of a paragraph
that is a [setext heading underline].

Reference link definitions are detected when a paragraph is closed;
the accumulated text lines are parsed to see if they begin with
one or more reference link definitions.  Any remainder becomes a
normal paragraph.

We can see how this works by considering how the tree above is
generated by four lines of Markdown:

``` markdown
> Lorem ipsum dolor
sit amet.
> - Qui *quodsi iracundia*
> - aliquando id
```

At the outset, our document model is just

``` tree
-> document
```

The first line of our text,

``` markdown
> Lorem ipsum dolor
```

causes a `block_quote` block to be created as a child of our
open `document` block, and a `paragraph` block as a child of
the `block_quote`.  Then the text is added to the last open
block, the `paragraph`:

``` tree
-> document
  -> block_quote
    -> paragraph
         "Lorem ipsum dolor"
```

The next line,

``` markdown
sit amet.
```

is a "lazy continuation" of the open `paragraph`, so it gets added
to the paragraph's text:

``` tree
-> document
  -> block_quote
    -> paragraph
         "Lorem ipsum dolor\nsit amet."
```

The third line,

``` markdown
> - Qui *quodsi iracundia*
```

causes the `paragraph` block to be closed, and a new `list` block
opened as a child of the `block_quote`.  A `list_item` is also
added as a child of the `list`, and a `paragraph` as a child of
the `list_item`.  The text is then added to the new `paragraph`:

``` tree
-> document
  -> block_quote
       paragraph
         "Lorem ipsum dolor\nsit amet."
    -> list (type=bullet tight=true bullet_char=-)
      -> list_item
        -> paragraph
             "Qui *quodsi iracundia*"
```

The fourth line,

``` markdown
> - aliquando id
```

causes the `list_item` (and its child the `paragraph`) to be closed,
and a new `list_item` opened up as child of the `list`.  A `paragraph`
is added as a child of the new `list_item`, to contain the text.
We thus obtain the final tree:

``` tree
-> document
  -> block_quote
       paragraph
         "Lorem ipsum dolor\nsit amet."
    -> list (type=bullet tight=true bullet_char=-)
         list_item
           paragraph
             "Qui *quodsi iracundia*"
      -> list_item
        -> paragraph
             "aliquando id"
```

## Phase 2: inline structure

Once all of the input has been parsed, all open blocks are closed.

We then "walk the tree," visiting every node, and parse raw
string contents of paragraphs and headings as inlines.  At this
point we have seen all the link reference definitions, so we can
resolve reference links as we go.

``` tree
document
  block_quote
    paragraph
      str "Lorem ipsum dolor"
      softbreak
      str "sit amet."
    list (type=bullet tight=true bullet_char=-)
      list_item
        paragraph
          str "Qui "
          emph
            str "quodsi iracundia"
      list_item
        paragraph
          str "aliquando id"
```

Notice how the [line ending] in the first paragraph has
been parsed as a `softbreak`, and the asterisks in the first list item
have become an `emph`.

### An algorithm for parsing nested emphasis and links

By far the trickiest part of inline parsing is handling emphasis,
strong emphasis, links, and images.  This is done using the following
algorithm.

When we're parsing inlines and we hit either

- a run of `*` or `_` characters, or
- a `[` or `![`

we insert a text node with these symbols as its literal content, and we
add a pointer to this text node to the [delimiter stack](@).

The [delimiter stack] is a doubly linked list.  Each
element contains a pointer to a text node, plus information about

- the type of delimiter (`[`, `![`, `*`, `_`)
- the number of delimiters,
- whether the delimiter is "active" (all are active to start), and
- whether the delimiter is a potential opener, a potential closer,
  or both (which depends on what sort of characters precede
  and follow the delimiters).

When we hit a `]` character, we call the *look for link or image*
procedure (see below).

When we hit the end of the input, we call the *process emphasis*
procedure (see below), with `stack_bottom` = NULL.

#### *look for link or image*

Starting at the top of the delimiter stack, we look backwards
through the stack for an opening `[` or `![` delimiter.

- If we don't find one, we return a literal text node `]`.

- If we do find one, but it's not *active*, we remove the inactive
  delimiter from the stack, and return a literal text node `]`.

- If we find one and it's active, then we parse ahead to see if
  we have an inline link/image, reference link/image, collapsed reference
  link/image, or shortcut reference link/image.

  + If we don't, then we remove the opening delimiter from the
    delimiter stack and return a literal text node `]`.

  + If we do, then

    * We return a link or image node whose children are the inlines
      after the text node pointed to by the opening delimiter.

    * We run *process emphasis* on these inlines, with the `[` opener
      as `stack_bottom`.

    * We remove the opening delimiter.

    * If we have a link (and not an image), we also set all
      `[` delimiters before the opening delimiter to *inactive*.  (This
      will prevent us from getting links within links.)

#### *process emphasis*

Parameter `stack_bottom` sets a lower bound to how far we
descend in the [delimiter stack].  If it is NULL, we can
go all the way to the bottom.  Otherwise, we stop before
visiting `stack_bottom`.

Let `current_position` point to the element on the [delimiter stack]
just above `stack_bottom` (or the first element if `stack_bottom`
is NULL).

We keep track of the `openers_bottom` for each delimiter
type (`*`, `_`), indexed to the length of the closing delimiter run
(modulo 3) and to whether the closing delimiter can also be an
opener.  Initialize this to `stack_bottom`.

Then we repeat the following until we run out of potential
closers:

- Move `current_position` forward in the delimiter stack (if needed)
  until we find the first potential closer with delimiter `*` or `_`.
  (This will be the potential closer closest
  to the beginning of the input -- the first one in parse order.)

- Now, look back in the stack (staying above `stack_bottom` and
  the `openers_bottom` for this delimiter type) for the
  first matching potential opener ("matching" means same delimiter).

- If one is found:

  + Figure out whether we have emphasis or strong emphasis:
    if both closer and opener spans have length >= 2, we have
    strong, otherwise regular.

  + Insert an emph or strong emph node accordingly, after
    the text node corresponding to the opener.

  + Remove any delimiters between the opener and closer from
    the delimiter stack.

  + Remove 1 (for regular emph) or 2 (for strong emph) delimiters
    from the opening and closing text nodes.  If they become empty
    as a result, remove them and remove the corresponding element
    of the delimiter stack.  If the closing node is removed, reset
    `current_position` to the next element in the stack.

- If none is found:

  + Set `openers_bottom` to the element before `current_position`.
    (We know that there are no openers for this kind of closer up to and
    including this point, so this puts a lower bound on future searches.)

  + If the closer at `current_position` is not a potential opener,
    remove it from the delimiter stack (since we know it can't
    be a closer either).

  + Advance `current_position` to the next element in the stack.

After we're done, we remove all delimiters above `stack_bottom` from the
delimiter stack.