Hans et al.,
I know you are probably constrained to define you GDM schema/schemata in XML
Schema. Are you using a tool to do this or are you doing it by hand?
Are you familiar with RELAX NG?  It is an alternative schema language for
XML produced under the auspices of OASIS  which is highly addictive.
James Clark, technical lead of the working group at the W3C that produced
XML 1.0, is chair of the RELAX NG technical committee.
I have produced a number of schemata in both XML Schema and RELAX NG which
validate against the same instance. It is an interesting exercise. In my
opinion, RELAX NG wins the contest hands down.
There is a tool, DTDinst , that can generate a RELAX NG schema from a
DTD. Jing is a readily available validator written in Java. 
I am not advocating that you drop XML Schema in favor of RELAX NG. However,
it would be nice to offer alternative schemata -- all in good time, of
P.S. I am on the RELAX NG technical committee, yes, but I promote it only on
the basis of its virtues.
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