Testing a nonnegative fixnum against 0
Zpf is exactly the same as zp, except that zpf is
intended for, and faster for, fixnum arguments. Its guard is specified with a
type declaration, (type (unsigned-byte 29) x). (See declare and
see type-spec.) Also see zp.
(defun zpf (x)
(declare (type (unsigned-byte 29) x))
(if (integerp x) (<= x 0) t))