Add interleave element (and deprecate sequence/@preserveOrder)

[joeytakeda]

Resolves #2154 by adding the interleave element and deprecating sequence/@preserveOrder

At the moment, I'm leaving this as draft as per @hcayless' observation that we should figure out how best to validate the defined content model to prevent overlap. IMO, we should probably warn about this in the prose and in the remarks for the element, but true validation will probably need to be left up to the ODD processor.

[joeytakeda]

Moving this to be ready for review, since I think the question of validity is probably something that needs to be handled by the processor (but have assigned mostly ATOP folks for review to see what they think about that)

[sydb]

Just to make the concern initially raised by @hcayless (on the ticket) echoed by @joeytakeda (here) explicit …

In RELAX NG it is not legal to have the same element occur in an interleave more than once. And because interleave “reaches in” to the patterns listed, this is hard to check for (other than by running a RELAX NG processor like jing … I just discovered that neither trang nor nxml-mode object).

So, for example, the following is not allowed in RELAX NG:

start = element blood { ( a & b & o & ab ) }

a = element a { empty }
b = element b { empty }
o = element o { empty }
ab = ( a | b )

And it does not matter whether the ab pattern is defined as an alternation (<rng:choice> or |), a sequence (<rng:group> or ,), or an interleave (<rng:interleave> or &). In all cases the reference to the pattern ab throws an error. (The message is “the element "b" can occur in more than one operand of "interleave"”, which I, for one, find confusing.)

[sydb]

I would prefer the issues I note be addressed before merge, but it is not a big deal. (And I may get to the easier ones myself, soon.)
That said, we do need a discussion on the fact that use of <interleave> in an ODD means that the RELAX NG generated by either the Stylesheets or by ATOP will not be transformable into DTD language.

[sydb]

Can-of-worms department.

(Note to @joeytakeda — most of the problem here is not from your improvement, but has been here all along and is just foregrounded by your change.)

This sentence does not mention that cardinality of the stuff inside <interleave>. But on thinking of how to re-word it, I realize that these sentences discuss the cardinality of references, when what we are talking about is the cardinality of the thing referenced. I am not sure what, if anything, we should do about this.

[sydb]

Given that a) <interleave> did not make it into the recent release, and b) use of @preserveOrder still works in the Stylesheets, I think the @validUntil date should be moved up to late 2025 or early 2026.

[tuurma]

F2F discussion in Buenos Aires: We should provide a <remark> in the <interleave> element spec to caution people working with elements belonging to multiple classes, or when using classRef.

[raffazizzi]

Subgroup @sydb @joeytakeda @raffazizzi suggest also creating a Stylesheet ticket to check validity of rng before continuing build in order to catch validation errors caused by interleave/classRef (and maybe more!)

[joeytakeda]

Stylesheets ticket created here: TEIC/Stylesheets#719 — for discussion at the next council meeting is whether the approval of this PR (e.g. the additional of interleave) is contingent on the validation process.