VMS Help PASCAL, Predeclared Routines, Arithmetic, XOR  *Conan The Librarian (sorry for the slow response - running on an old VAX)

  The XOR function returns a  value  (of  the  same  type  as  the
  parameters)  of  a  binary logical exclusive-OR operation on two
  specified parameters.

  Syntax:

     XOR( p1,p2 )

  The 'p1' and 'p2' parameters must be of the same type  and  must
  be of either the BOOLEAN or SET types.