(letrec
   (fix f x = f (fix f) x)
   (map f xs = case xs of
                 (Cons x xs -> Cons (f x) (map f xs))
                 (Nil       -> Nil))

   (mul a b = case b of
                (0 -> 0)
                (n -> add a $ mul a $ add b $ negate 1))

   (factorial a = case a of
                    (0 -> 1)
                    (a  -> mul a $ factorial $ add a $ negate 1))
in
(let (fac = fix $ \ rec a -> case a of
                    (0 -> 1)
                    (a  -> mul a $ rec $ add a $ negate 1))
in
 fac 7))