xmi schema validation

Hi,

I'm interested in creating a program that generates xmi files that can be read 
by umbrello... to be able to verify that when I generate makes sense in the 
context of the format I'd like to be able to do validation of it via an xmi 
schema..

I've done some google searches and found that www.omg.org has a whole bunch of 
pdfs describing the purpose of xmi and various possible applications of it, 
etc etc etc, but I can't find xsd files there (or elsewhere) to perform 
simple file validation...

does anyone know where I could find xmi schema files?

thx

-Jamie

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