> if 'rel-id' were written 'id' in the source tree (and you change the 
> match accordingly) this would work.

no even if the attribute had the same name, it would be a different node
with different identity. 

> Unfortunately, the operator 
> 'intersect' is defined to compare nodes only, and cannot compare 
> sequences of strings... :S


= allows you do do value-based intersection (of strings for example)
if $s1 and $s2 are sequences of strings then 
$s1[.=$s2] 
is the sequence of strings that are common to $s1 and $s2.

David