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| ANSI Common Lisp 14 Conses 14.2 Dictionary of Conses
14.2.28 butlast, nbutlast | Function
  Syntax: butlast list `&optional` n result-list nbutlast list `&optional` n result-list Arguments and Values: list - a list, which might be a dotted list but must not be a circular list. n - a non-negative integer. result-list - a list. Description: butlast returns a copy of list from which the last n conses have been omitted. If n is not supplied, its value is 1. If there are fewer than n conses in list, nil is returned and, in the case of nbutlast, list is not modified. nbutlast is like butlast, but nbutlast may modify list. It changes the cdr of the cons n+1 from the end of the list to nil. Examples:
       (setq lst '(1 2 3 4 5 6 7 8 9))  (1 2 3 4 5 6 7 8 9)
       (butlast lst)  (1 2 3 4 5 6 7 8)
       (butlast lst 5)  (1 2 3 4)
       (butlast lst (+ 5 5))   NIL
       lst  (1 2 3 4 5 6 7 8 9)
       (nbutlast lst 3)  (1 2 3 4 5 6)
       lst  (1 2 3 4 5 6)
       (nbutlast lst 99)   NIL
       lst  (1 2 3 4 5 6)
       (butlast '(a b c d))  (A B C)
       (butlast '((a b) (c d)))  ((A B))
       (butlast '(a))   NIL
       (butlast nil)   NIL
       (setq foo (list 'a 'b 'c 'd))  (A B C D)
       (nbutlast foo)  (A B C)
       foo  (A B C)
       (nbutlast (list 'a))   NIL
       (nbutlast '())   NIL
  Exceptional Situations: Should signal an error of type type-error if list is not a proper list or a dotted list. Should signal an error of type type-error if n is not a non-negative integer. Notes:
       (butlast list n) ==(ldiff list (last list n))
Allegro CL Implementation Details: None.

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