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From: "Michael Kay" <firstname.lastname@example.org>
> Is there thought going on regarding constraints on collections (when they
> are defined)? I may want to have all
> invoice numbers in a collection be unique.
> I haven't seen any proposals that address that. Certainly, XML Schema
> doesn't provide any cross-document validation.
The XLlinkIt validation language at www.xlinkit.com is aimed squarely at this
kind of problem, I think. (Christian Nentwich who sometimes posts on this list is,
I believe, one of the programmers involved in it.)
Schematron can do some kinds of inter-document validation, in particular
when there is a link in one document to another, e.g. to a controlled vocabulary.
But it lacks a for-each operator, and so is weak for collections. Xpath 2
may help in this, I don't know.
I gave up trying to figure out a Schematron expression to do this, but undoubtedly
something inefficient based on counting and the document() function could be