plus = fix [plus] lam [x] lam [y]
         case x y ([x] s (app (app plus x) y)).

minus = fix [minus] lam [x] lam [y]
         case x z ([x] case y x ([y] (app (app minus x) y))).

mult = fix [mult] lam [x] lam [y]
         case x z ([x] (app (app plus y) (app (app mult x) y))).