using a combination of exslt set functions (pure xslt)

http://www.exslt.org/set/functions/difference/index.html

you can achieve this in XSLT 1.0

In XSLT 2.0 one has access to these XPATH 2 funcs e.g. set difference,
intersection and union operators.

http://www.saxonica.com/documentation/expressions/except.html

gl, Jim Fuller