nset-difference 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-difference is the destructive version of set-difference. It may destroy list-1.

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. The first argument to the :test or :test-not function is the part of an element of list-1 that is returned by the :key function (if supplied); the second argument is the part of an element of list-2 that is returned by the :key function (if supplied).

If :key is supplied, its argument is a list-1 or list-2 element. The :key function typically returns part of the supplied element. If :key is not supplied, the list-1 or list-2 element is used.

An element of list-1 appears in the result if and only if it does not match any element of list-2.

There is no guarantee that the order of elements in the result will reflect the ordering of the arguments in any particular way. The result list may share cells with, or be eq to, either of list-1 or list-2, if appropriate.

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

;; Remove all flavor names that contain "c" or "w".
 (set-difference '("strawberry" "chocolate" "banana"
                  "lemon" "pistachio" "rhubarb")
          '(#\c #\w)
          :test #'(lambda (s c) (find c s)))
 ("banana" "rhubarb" "lemon")    ;One possible ordering.