Support in myst for multiple directive arguments

[mmcky]

In Sphinx the index generating directive has the ability to use multiple arguments such as:

.. index::
   single: execution; context
   module: __main__
   module: sys
   triple: module; search; path

https://www.sphinx-doc.org/en/master/usage/restructuredtext/directives.html#directive-index

I am not sure how to specify this use of multiple arguments in myst. As far as I am aware arguments need to be on the same line as the directive such as

```{index} single: execution; context
```

@chrisjsewell is there a way to specify multiple options at the moment?

---

I also note that in myst we use a similar syntax to specify options (albeit within the yaml headers only).

```{code-block}
---
class: <some class>
----
```

I don't see this as a big issue -- just something to be aware of. I think the yaml demarcators are a clear indicator that your specifying options.

[chrisjsewell]

have you tried just running?

```{index}
single: execution; context
module: __main__
module: sys
triple: module; search; path
```

[mmcky]

sorry should have mentioned that:

Using

```{index}
single: Python; IO
```

produces the following warning:

/Users/matthewmckay/repos-collab/lecture-python-programming.myst/source/python_essentials.md:274: WARNING: Directive 'index': 1 argument(s) required, 0 supplied

single: Python; IO

[chrisjsewell]

what about?

```{index} single: execution; context
module: __main__
module: sys
triple: module; search; path
```

I just seem to remember "fixing" multi-line argument strings, for use by sphinx-tabs

[mmcky]

Nice idea -- hadn't tried that continuation from the options line. Tried:

```{index} single: Python; Runtime
single: Julia
```

but get No content permitted:

/Users/matthewmckay/repos-collab/ebp-test-projectstructure/issues/index-directive/chapter3.md:3: WARNING: Directive 'index': No content permitted

single: Julia

[mmcky]

I was thinking we could have a line continuation marker, something like:

```{index} >
single: Python; Runtime
single: Julia
```

with content (if any in the future is needed) demarcated by a line break

```{index} >
single: Python; Runtime
single: Julia

<content>
```

and with options

```{index} >
single: Python; Runtime
single: Julia
---
name:
---
```

or

```{index} >
single: Python; Runtime
single: Julia
:name: <targetname>
```

but not sure if that will cause issues on the parser side and the last example is a little confusing with : the only indicator of difference between argument and option.

[chrisjsewell]

the > could lead to issues, if you actually want to use > as the argument.
I think the logic would be something like: if the directive has a required argument, and the argument line is blank, try to take it from the next line(s) until the options start, or there is a blank line (I guess this is what docutils actually does)