RE: Grouping-by appropriate values from external docu

> Note that whenever you have a construct like
> 
> [@id=current()/@pr_id]
> 
> "use a key" should pop automatically in to your head.

Either "use a key", or "use a product whose optimizer does automatic
indexing" - like Saxon-SA (shameless plug).

(I haven't checked that it works in this case, but it's a textbook example,
so it should)

Michael Kay
http://www.saxonica.com/