Recognizer for positive naturals and nil.
This is like an option type; when x
satisfies maybe-posp, then either it is a positive, natural number or
(defun maybe-posp$inline (x)
(declare (xargs :guard t))
(or (not x) (posp x)))
(equal (maybe-posp x)
(or (and (integerp x) (< 0 x)) (not x)))