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From: "Jeff Lowery" <Jeff.Lowery@creo.com>
> > 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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