[relax-ng] Identity constraints

Another area where I got some feedback was with respect to identity
constraints.  There seemed to be some agreement that identity constraints
didn't belong in XML Schema.  The database constituency didn't really see
the need for anything -- you can easily just use XQuery to express any
constraint you want. However, outside the database constituency, there did
seem to be a feeling that something lighter-weight than full XQuery was
needed.  I would summarize the desired level of functionality as a bit more
than what's in XML Schema, but with a strong requirement that it must be
streamable.

James