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Clemens Vasters: Complete agreement may be a problem, but agreement on a composite, well-defined subset of things that both parties are interested in and understand is not. Pardon me, but did you say both? BOTH? BOTH!!!??? Works well 1:1, but how about nnn:mmmmmm?
And then later: Loose and tight coupling can happily coexist. I was more thinking of other ways. Ones that do not require existing instance documents to be invalidated when an XML schema is only extended. For further reading, see the XML Schema Versioning Best Practice document produced by the MITRE Corporation and the xml-dev list group.