nset-exclusive-or list-1 list-2 &key key test test-not    result-list

list-2 - a proper list.

test - a designator for a function of two arguments that returns a generalized boolean.

test-not - a designator for a function of two arguments that returns a generalized boolean.

key - a designator for a function of one argument, or nil.

result-list - a list.

nset-exclusive-or is the destructive version of set-exclusive-or.

For all possible ordered pairs consisting of one element from list-1 and one element from list-2, the :test or :test-not function is used to determine whether they satisfy the test.

If :key is supplied, it is used to extract the part to be tested from the list-1 or list-2 element. The first argument to the :test or :test-not function is the part of an element of list-1 extracted by the :key function (if supplied); the second argument is the part of an element of list-2 extracted by the :key function (if supplied). If :key is not supplied or nil, the list-1 or list-2 element is used.

The result contains precisely those elements of list-1 and list-2 that appear in no matching pair.

The result list of set-exclusive-or might share storage with one of list-1 or list-2.

 (setq lst1 (list 1 "a" "b")
       lst2 (list 1 "A" "b"))  (1 "A" "b")
 (set-exclusive-or lst1 lst2)  ("b" "A" "b" "a")
 (set-exclusive-or lst1 lst2 :test #'equal)  ("A" "a")
 (set-exclusive-or lst1 lst2 :test 'equalp)   NIL 
 (nset-exclusive-or lst1 lst2)  ("a" "b" "A" "b") 
 (setq lst1 (list (("a" . "b") ("c" . "d") ("e" . "f"))))
 (("a" . "b") ("c" . "d") ("e" . "f"))
 (setq lst2 (list (("c" . "a") ("e" . "b") ("d" . "a"))))
 (("c" . "a") ("e" . "b") ("d" . "a")) 
 (nset-exclusive-or lst1 lst2 :test #'string= :key #'cdr)
 (("c" . "d") ("e" . "f") ("c" . "a") ("d" . "a")) 
 lst1  (("a" . "b") ("c" . "d") ("e" . "f"))
 lst2  (("c" . "a") ("d" . "a")) 

Since the nset-exclusive-or side effect is not required, it should not be used in for-effect-only positions in portable code.