RDF for metadata

One suggested role for RDF is as metadata for XML.  (The "semantics"
of XML as some have imprecisely said.)  The idea is that RDF (or
RDF+rules, or RDF+DAML+OIL) would be used to make assertions that
constrain the meaning of XML tags.  E.g., if there is a 'family' tag
in some XML vocabulary, so we could write

   <family>
      <father><person> ...</person></father>
      <mother><person> ...</person></mother>
      <children>
          <person> ...</person>
          <person> ...</person>
      </children>
   </family>

then we could give constraints on these symbols.  (No child has more
than one mother, or the like.)

Has anyone actully done this for any existing XML vocabulary?  Are
there pointers to such efforts?  I'm having trouble visualizing
exactly how RDF would do this job.

                                             -- Drew McDermott