; a molecule is not an atom and must consist of 2 or more atoms
; atom = (not cons)
(deftype molecule ()
  `(cons atom atom))

(typep '(a . b) 'molecule) ; T
(typep '(a) 'molecule) ; T
(typep '((a) . b) 'molecule) ; NIL
(typep '(a b) 'molecule) ; NIL
(typep "a" 'molecule) ; NIL
(typep nil 'molecule) ; NIL
(typep (lambda ()) 'molecule) ; NIL