Undecidability of a Schematron problem

Recently, I was reading about the Post Correspondence Problem
(PCP), which is undecidable (i.e., unsolvable). I realized
that the undecidability of the PCP could be used to prove that
the following problem is undecidable:

     Given a Schematron schema, is there an XML document that
     conforms to this schema?

If you're interested in reading the proof and a description
of the PCP, then please visit
http://www.unidex.com/scp/index.htm.

Best regards,

Bob

<sig name    = 'Bob Lyons'
     title   = 'XML and Java Consultant'
     company = 'Unidex, Inc.'
     phone   = '+1-732-975-9877'
     email   = ''
     url     = 'http://www.unidex.com/'
     product = 'XML Convert: transforms flat files to XML and vice versa' />