data N = Z | S (n:: N)

zero :: N = Z
one  :: N = S zero
two  :: N = S one
three:: N = S two


data NatList = nil | cons (n:: N) (l:: NatList)

map (f :: N -> N)
    (l:: NatList)
    :: NatList
    = case l of
      (nil      )->
        nil
      (cons n l')->
        cons (f n) (map f l')

test :: NatList
     = map (\(n::N)-> S n) 
           (cons two (cons three nil))

showTest :: NatList
         = {!test !}