[OT][slightly] Converting Schema to DTD

Is there a simple way to convert an XML Schema file to a DTD?  I've
never been able to find a DTD for Visio's XML, but I did find a schema
file.  Trouble is, Emacs's XML package depends on the DTD to figure
out what's what.

It's not vital, but it would make life a little easier...
--
John Breen