414: Attempt to implement expanding the allowed character repertoire

[ndw]

Fix #414

This PR addresses ACTION QT4CG-036-01 on me.

[michaelhkay]

(a) I think it might be worth emphasizing that unpaired surrogates can never be permitted characters (because they are not Unicode characters) (for example in codepoints-to-string())
(b) It might be worth saying something in the serialization spec - perhaps adding a serialization error condition?

[ndw]

Yes, good suggestions.

[Arithmeticus]

For consistency, the type of edits applied in this commit also need to be applied to the following (my comments based upon the qt4cg diff html files for this PR):

• 1.8.1 -- the data model edits need to be applied to their summary counterparts.
• 1.8.1 first note: these substantive restrictions would be better placed in the data model document, particularly material referencing the surrogate blocks.
• 1.8.1 second note: I think the revisions mean that the set of codepoints and the set of characters are coterminous, no? (With the proviso that an implementer may choose to not make them so.) Or maybe I misunderstand.
• first note for fn:unparsed-text-available(): "no characters that are invalid in XML"

[ndw]

I managed to lose all my commits and GitHub thoughtfully closed the issue for me. :-/

I've reapplied the changes and reopened the PR.

@michaelhkay I've added a note about unpaired surrogates. I don't believe any new errors are required in serialization, there's already an error SER0006 for "character not allowed in this version of XML" which I think applies.

[Arithmeticus]

@ndw You may also want to adjust the language in xpath-datamodel.xml at lines 932-934:

This means, for example, that the xs:string data type should (at the time of writing) support the set of characters defined by the Char production in XML 1.1 Second Edition.

The relative parenthetical clause gives me slight chills, as well.

[ndw]

The CG agreed to merge this issue at meeting 043