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| ANSI Common Lisp 14 Conses 14.2 Dictionary of Conses
14.2.44 adjoin | Function
  Syntax: adjoin item list `&key` key test test-not new-list Arguments and Values: item - an object. list - 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. new-list - a list. Description: Tests whether item is the same as an existing element of list. If the item is not an existing element, adjoin adds it to list (as if by cons) and returns the resulting list; otherwise, nothing is added and the original list is returned. The test, test-not and key affect how it is determined whether item is the same as an element of list. For details, see Examples:
       (setq slist '())   NIL
       (adjoin 'a slist)  (A)
       slist   NIL
       (setq slist (adjoin '(test-item 1) slist))  ((TEST-ITEM 1))
       (adjoin '(test-item 1) slist)  ((TEST-ITEM 1) (TEST-ITEM 1))
       (adjoin '(test-item 1) slist :test 'equal)  ((TEST-ITEM 1))
       (adjoin '(new-test-item 1) slist :key #'cadr)  ((TEST-ITEM 1))
       (adjoin '(new-test-item 1) slist)  ((NEW-TEST-ITEM 1) (TEST-ITEM 1))
  Exceptional Situations: Should be prepared to signal an error of type type-error if list is not a proper list. See Also: pushnew, Section 3.6 Traversal Rules and Side Effects Notes: The `:test-not` parameter is deprecated.
       (adjoin item list :key fn)
         ==(if (member (fn item) list :key fn) list (cons item list))
Allegro CL Implementation Details: None.

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