Listp
Recognizer for (not necessarily proper) lists
(listp x) is true when x is either a cons pair or
is nil.
Listp has no guard, i.e., its guard is t.
Listp is a Common Lisp function. See any Common Lisp documentation
for more information.
Function: listp
(defun listp (x)
(declare (xargs :guard t))
(or (consp x) (equal x nil)))
