(defun nod(a b)
 (cond
  ((= a b) a)
  ((> a b) (nod (- a b) b) )
  (1 (nod a (- b a) ) )
 )
)

(nod 3 6)
(nod 5 7)
(nod 128 12)
(nod 63 21)
(system)