Test whether an integer is even
(evenp x) is true if and only if the integer x is even.
Actually, in the ACL2 logic (evenp x) is defined to be true when x/2
is an integer.
The guard for evenp requires its argument to be an integer.
Evenp is a Common Lisp function. See any Common Lisp documentation
for more information.
(defun evenp (x)
(declare (xargs :guard (integerp x)))
(integerp (* x (/ 2))))