The sorts of properties which can be expressed involve:
Potential benefit from descriptors is determined by the ability of the descriptor language to express appropriate constraints. Therefore, we need a powerful language for expressing (and a broad interpretation of what are ) datatype constraints.
Why shouldn't ``an IDREF whose target is an element of type foo'' be a type?
Schema Datatype specification actually defines a standard in which we can write trans-vendor invariants for Data Verification.
This is NOT program verification.