Re: XML Schema

> Then you want to impose a constraint that at least one occurrence of
> element
> x must have a specified attribute value.  This is an example of what are
> sometimes called co-occurrence constraints.  Such constraints cannot be
> defined using XML Schema.

This would be trivial to implement using embedded Schematron rules. See [1]
for an example.

Cheers,
/Eddie

[1] http://lists.w3.org/Archives/Public/xmlschema-dev/2001Nov/0049.html