#555 improve constraint constraints

[Syd Bauman]

Per e-mail exchange “Please take a look at this bug”, 2015-01-22/29 on TEI Council mailing list:
1) Add XSLT to list of @scheme checked for in <constraintspec>s constraintSpecs.
2) Make actual tests simpler (use 'ne' as opposed to not())</constraintspec>

[Lou Burnard]

Supporting XSLT is really quite complex. Suggest that this should be removed.

[Sebastian Rahtz]

I'd suggest that as a matter of principle changes to ODD should only be put in when accompanied by full implementation. If there are documented features in the language which don't work in the processing, thats not nice for users. I do think ODD is different in that respect from the rest of TEI, as we claim to offer a took which reads the stuff.

so scheme="xslt" falls into the category

IMHO

[Syd Bauman]

Well I’m not sure I agree either that constraint with XSLT is that hard to implement or that we should only support values of constraintSpec/@scheme for which we have a full implementation. Luckily, this FR, (which is that if constraintSpec/@scheme is xsl then the content of its child <constraint> should, in fact, be XSLT) has nothing to do with either of those issues.

Council decided yesterday this is a good idea, so I’ve committed it at r13229.