You are so helpful, as always.
Joelle

If $staff is all the <staff> elements in the staff file, and $psr is all the
<project_staff_role> elements, then the $staff who are not in the selected
project are:

$staff[not(@staff_id = $psr[@project_id=$pid]/staff/@staff_id)]

Regards,

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