Add support for adding labels (ids) to steps

[ExE-Boss]

Needed to avoid tc39/ecma262#1835 from happening again.

Proposed syntax:

Allow <label_name>: in ordered lists, where <label_name> matches the regular expression:

^[a-zA-Z_$][a-zA-Z0-9_$]*$

Example:

<emu-clause id="sec-assertfoo" aoid="AssertFoo">
	<h1>AssertFoo( _val_ )</h1>

	<p>The AssertFoo algorithm ensures that _val_ is an object with an [[IsFoo]] internal slot, by only returning in <emu-xref aoid="AssertFoo" step="Is_Foo_$1"></emu-xref>.</p>

	<emu-alg>
		1. Assert: Type(_val_) is Object.
		1. If ? _val_.[[IsFoo]] is *true*
			Is_Foo_$1:
			1. Return _val_
		1. Throw a *TypeError* exception.
	</emu-alg>
</emu-clause>

[step Is_Foo_$1] would be replaced with an anchor link linking to the labeled step with the HTML step ${num}, where ${num} is the number of the step.

See also:

• Support referencing algorithm steps ecmarkup#192

[ljharb]

Would this work with a step used anywhere in the entire document? if so, perhaps it should be namespaced under the name of the abstract operation?

[bakkot]

I went with a slightly different syntax in #74: 1. [label="foo"] Step. I didn't namespace them because emcarkup doesn't really have a notion of the abstract operation associated with an algorithm; algorithm steps can appear wherever.

[ljharb]

Does that mean the label must be unique across the whole document?

[bakkot]

Yup.

That's not such a constraint, though; you don't really end up referring to individual steps that often. Ecma262, which is not a small document, currently has 59 steps which are referred to.