Re: combine

>Sounds interesting. A "computationally non-trivial nice-to-have" would
   >be combine="restrict". This would be the same as replace, but it would
   >be an error if the new content model was not a strict subset of the
   >current content model.

I agree this would be nice to have, but I think it is too much to require of implementations. I think the way too handle this is to use combine="replace", and then use an attribute in another namespace to indicate that it is intended to be a restriction.

James