Adding #xref inside #creator

[Oscar Levin]

At today's drop-in, Josh G. asked if it was possible to include a citation inside the #creator element in a #theorem.  It looks like the schema doesn't allow #xref as a child of #creator (although many other tags are allowed).  

I'd be happy to take a run at this, but wanted to make sure there wasn't a good reason not to. 

[Oscar Levin]

I guess it works fine, but is just not in the schema.