Element content models with valList should not have any other content

[martindholmes]

This arises out of the Stylesheets working group meeting on TEIC/Stylesheets#365.

We notice that the content model of <content> allows <valList> to be in alternation or sequence with other content model parts such as <elementRef> or <sequence>. We believe that such constructions are fundamentally mysterious -- what does it mean to say that something has a closed value list but then has a text node, for instance? -- and that practically speaking, using a <valList> in an element's content model only makes sense if that's the only thing you want to be in there.

So we think that the content model of <content> should be changed such that it may either be <valList> or the current model without <valList>.

[lb42]

I agree that it's hard to see what it would mean to combine a valList in sequence with other members of model.contentPart, but the current model is intended to permit an alternation, which doesn't seem unreasonable.

[martindholmes]

Council F2F says that such combinations are impossible to process into a workable schema, and there is no imaginable use-case for them, so we should disallow them.

[sydb]

1. removed <valList> from model.contentPart
2. put <valList> directly into content model of <content> and <alternate> — but such that it cannot be in sequence with other stuff
3. (Did not put <valList> directly into content model of <sequence>.)

Tested locally, seemed fine. Thus closing in anticipation that Mr. Jenkins is happy.