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I once had a variation on this: it was for new reports where each
document had to have elements for <what>, <where>, <when>, <who> (and
<how>), which a journalism professor was teaching as the essential
elements that every professional story needed to provided.
So the constraints: five elements, where the document needed to have at
least one of each in any order. The killer? The elements could be
anywhere in the news report.
Obviously Schematron can do this trivially
<rule context="/">
<assert test="//what">A document should have at least one what
</assert>
<assert test="//where">A document should have at least one where
</assert>
<assert test="//when">A document should have at least one when
</assert>
<assert test="//who">A document should have at least one who </assert>
<assert test="//how">A document should have at least one how</assert>
</rule>
but it is not something that grammars (or, at least, grammars with
non-path particles) can do.
Even in the case of XSD 1.1 assertions, which are a great addition
(perhaps in the way that a cheesecake is great after a 50 course meal of
crackers) it is difficult, because these are not constraints that
belong to types. They are constraints of the document of a class rather
than of any elements: so in XSD you would have to artificially tie
these to, say, a top-level element. In XSD it cannot be expressed as a
pattern that exists in isolation from other particulars of the document.
The pattern concept is more general than the type concept.
Cheers
Rick Jelliffe
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