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| ANSI Common Lisp 14 Conses 14.2 Dictionary of Conses
14.2.45 pushnew | Macro
  Syntax: pushnew item place `&key` key test test-not new-place-value Arguments and Values: item - an object. place - a place, the value of which is 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-place-value - a list (the new value of place). Description: pushnew tests whether item is the same as any existing element of the list stored in place. If item is not, it is prepended to the list, and the new list is stored in place. pushnew returns the new list that is stored in place. Whether or not item is already a member of the list that is in place is determined by comparisons using `:test` or `:test-not`. The first argument to the `:test` or `:test-not` function is item; the second argument is an element of the list in place as returned by the `:key` function (if supplied). If `:key` is supplied, it is used to extract the part to be tested from both item and the list element, as for adjoin. The argument to the `:key` function is an element of the list stored in place. The `:key` function typically returns part part of the element of the list. If `:key` is not supplied or nil, the list element is used. For information about the evaluation of subforms of place, see Section 5.1.1.1 Evaluation of Subforms to Places. It is implementation-dependent whether or not pushnew actually executes the storing form for its place in the situation where the item is already a member of the list held by place. Examples:
       (setq x '(a (b c) d))  (A (B C) D)
       (pushnew 5 (cadr x))  (5 B C)
       x  (A (5 B C) D)
       (pushnew 'b (cadr x))  (5 B C)
       x  (A (5 B C) D)
       (setq lst '((1) (1 2) (1 2 3)))  ((1) (1 2) (1 2 3))
       (pushnew '(2) lst)  ((2) (1) (1 2) (1 2 3))
       (pushnew '(1) lst)  ((1) (2) (1) (1 2) (1 2 3))
       (pushnew '(1) lst :test 'equal)  ((1) (2) (1) (1 2) (1 2 3))
       (pushnew '(1) lst :key #'car)  ((1) (2) (1) (1 2) (1 2 3))
  Side Effects: The contents of place may be modified. See Also: push, adjoin, Section 5.1 Generalized Reference Notes: The effect of
       (pushnew item place :test p)
  is roughly equivalent to
       (setf place (adjoin item place :test p))
except that the subforms of `place` are evaluated only once, and `item` is evaluated before `place`. Allegro CL Implementation Details: None.

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