intersect3 lista listb
              Procedure to intersects two lists, returning a list
              containing three lists:  The first list returned is
              everything in lista that wasn't in listb.  The sec-
              ond list  contains  the  intersection  of  the  two
              lists, and the third list contains all the elements
              that were in  listb  but  weren't  in  lista.   The
              returned lists will be sorted.

              This procedure is provided by Extended Tcl.