Major Section: ACL2-BUILT-INS
(Butlast l n) is the list obtained by removing the last n
elements from the true list l. The following is a theorem
(though it takes some effort, including lemmas, to get ACL2 to prove
it).
(implies (and (integerp n)
(<= 0 n)
(true-listp l))
(equal (length (butlast l n))
(if (< n (length l))
(- (length l) n)
0)))
For related functions, see take and see nthcdr.
The guard for (butlast l n) requires that n is a nonnegative
integer and lst is a true list.
Butlast is a Common Lisp function. See any Common Lisp
documentation for more information. Note: In Common Lisp the
second argument of butlast is optional, but in ACL2 it is
required.
To see the ACL2 definition of this function, see pf.
