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From: "Jeff Lowery" <Jeff.Lowery@c...> > > I'm trying to understand why this is so: You mean, why is this a hard problem? See http://www.dcs.bbk.ac.uk/~ptw/icdt-talk.pdf. > > If a document validator can know if a field occurrence is > > optional in an instance's corresponding content model, then > > how is it that a schema validator cannot know that a declared > > key field is optional in its corresponding content model? >> > > The only thing I can think of that's different is that the > > document validator may be able to narrow down the applicable > > content models based on components that've already be read > > from the document. A document validator only has to check whether a given instance satisfies the constraints in the schema. The other question requires the processor to consider all possible instances. A development tool might offer additional checking like this (though it is kind of a rathole), since the answers are primarily useful for debugging schemas, but such checks don't make validation more powerful and probably shouldn't be required by a schema language. There is ample precedent, e.g., <!ELEMENT foo (foo)> Bob
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