(push item res)))

 	     res)))))

 ;;; Destination and source are setf-able and many-evaluable.  Sets the source

 ;;; to the cdr, and "conses" the 1st elt of source to destination.

 ;;;

              (process (nth-value 1 (gethash (apply-key key (car list1)) hashtable))))

             (t

              (process (with-set-keys (member (apply-key key (car list1)) list2)))))

       res)))

 (defun nset-difference (list1 list2 &key key

              (process (nth-value 1 (gethash (apply-key key (car list1)) hashtable))))

             (t

              (process (with-set-keys (member (apply-key key (car list1)) list2)))))

       res)))

 (defun set-exclusive-or (list1 list2 &key key

                          (test #'eql testp) (test-not nil notp))

   "Return new list of elements appearing exactly once in LIST1 and LIST2."