How to use p:compare

I got this off the xproc list and I'm having trouble figuring out what
the right answer is.

What's the easiest way to write a step that compares two documents and
returns one of them if they're the same? If they're different, it
throws an error

                                        Be seeing you,
                                          norm

-- 
Norman Walsh <ndw@nwalsh.com> | Never contend with a man who has
http://nwalsh.com/            | nothing to lose.-- Gracián