RE: keyref with empty sets

> So, no keyref selections, no key selections, no problem. 

Hmmm.  Then identity constraints could be used to enforce the
HTML requirement that <A> elements not be nested.  Just define
a key constraint for <A> with select=".//A" and 
field="@SomeInvalidAttributeName".  If there's no nested <A>, 
then there's no error.  If there is a nested <A>, there's
an error whether it does or doesn't have the invalid attribute.

Just an idea.

xan