RE: requesting XML records

> The issue here seems to be, what do we mean by one schema 
> being "derived from" another?  Is there a ready-made answer 
> to this from the world of XML Schema?  Or do we have to 
> invent such a notion?

Not a direct mapping - in XML Schema we have redefines, import and
substitution groups.

Roughly speaking these work as:

Redefines - in this case I say that I take schema A, but for a
particular element x I say what structure I what it to be rather than
the structure given in schema A.

Import - If I import schema A into schema B, then at any point in the
structure I'm defining for schema B, I can introduce a structure
references in schema A

Substitution groups - I import Schema A, but say that for a particular
element x in schema A, I can in fact substitute an element y in defined
in schema B

Matthew