Cross-referencing

[Matthias Clasen <mclasen@sun2.mathematik.uni-freiburg.de>]

Hello all,
I have a simple question: Is it possible to process cross-references with
just the Core Query Language currently present in Jade ?

The situation is the following: My document looks like

<theorem id=foo> ... </theorem>

<proof of=foo> ... </proof>

(`id' is an ID attribute, `of' an IDREF)

I could something like 

(element THEOREM
  (make paragraph
    (literal (append-string "Theorem "
                            (number->string (child-number))
                            "."))
    (process-children)))

to give my theorem a running number. But how do I generate the 
corresponding number for the proof ? My study of the standardbrought up
the following solution:

(element PROOF
  (make paragraph
    (literal "Proof of "
             (number->string (child-number (referent)))
             ":"))
    (process-children)))

but Jade chokes on `referent', which is not part of the Core Query 
Language. Any ideas?
 
Thanks in advance, Matthias

PS The next task will be to omit the number if the proof immediately 
follows the theorem. That should be easier.