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This comment appears in http://lists.w3.org/Archives/Public/www-xml-schema-comments/2009JanMar/. This bugzilla entry is so that the WG can track whether or not it has been addressed. It may be a duplicate. Apologies if it is. http://lists.w3.org/Archives/Public/www-xml-schema-comments/2009JanMar/0143.html
The WG discussed this issue at its telcon today. The results have been transmitted to the originator of the issue by email archived at http://lists.w3.org/Archives/Public/www-xml-schema-comments/2009AprJun/0115.html the salient content of which reads: Dear Herr Grundmann, Many thanks for your interest in XSD 1.1, and for your comment on the expressive power of the XPath subset defined for identity constraints. The XML Schema WG has created issue 6706 in the W3C instance of Bugzilla (http://www.w3.org/Bugs/Public/show_bug.cgi?id=6706) to track the issue you raise. We discussed this issue during our meeting today. The consensus in the WG was that the best way to handle this issue at this time is to classify it as a request for enhancement, and to recommend it for consideration in a future version of XSD. (The current version of XSD is, the WG felt, too far advanced to be able to take this suggestion into consideration in XSD 1.1.) In the meantime, some WG members suggested, it may be possible to use the assertions construct of XSD 1.1 to enforce integrity constraints of the type you have sketched in your example. Accordingly I am closing this issue with a disposition of LATER. Please let us know if you agree with this resolution of your issue, by adding a comment to the issue record and changing the Status of the issue to Closed. Or, if you do not agree with this resolution, please add a comment explaining why. If you wish to appeal the WG's decision to the Director, then also change the Status of the record to Reopened. If you wish to record your dissent, but do not wish to appeal the decision to the Director, then change the Status of the record to Closed. (Alternatively, if using Bugzilla is not convenient, you can just reply to this email.) If we do not hear from you in the next ten days or so, we will assume you agree with the WG decision. --- I'm updating the Bugzilla record accordingly.
For the record, I note here that I have received private mail from the originator of this comment saying (inter alia) "I understand and agree with the working group's decission not to add the proposed improvement to XML Schema in the current state."