Possible small RDDL enhancement

[Tim Bray <tbray@textuality.com>]

It dawns on me that it might be useful for a RDDL to declare
what namespace or mime-type it claims to be describing.  Would
it make sense to create a new optional element type, <rddl:class>, 
with attributes ns= and/or media-type=, which could be used for
this purpose?  I was thinking of using attributes, but sometimes
a rddl might be used to describe more than one class of documents.

*If* this is a good idea, the element might as well go in 
<html:head>, right?  -Tim