detect if relax-ng schema has validation that xsd cannot support?

Hi,

I'm wondering if there are any tools out there, preferably open source
but actual products are good as well, that can determine if a Relax-NG
schema validates structures that cannot be validated by XML Schema.

Actually given that Schematron can be written to support things that
can be done via XML Schema I wonder if there is such a tool for
schematron, given that they are very different languages conceptually
and I suppose the relative complexity of analysing without running a
schematron schema I would suppose the answer to this one would be
'no'.

MVH
Bryan Rasmussen