data W = Constr (A -> W) (B -> C -> W) D W

foldW :: ((A -> x) -> (B -> C -> x) -> D -> x -> x)
      -> W
      -> x
foldW f (Constr aw bcw d w) =
    f (a -> foldW f (aw a))
      (b c -> foldW f (bcw b c))
      d
      (foldW f w)

recW :: ((A - W) -> (A -> x) -> (B -> C -> W) -> (B -> C -> x) -> D -> W -> x -> x)
     -> W
     -> x
recW f (Constr aw bcw d w) =
    f aw
      (a -> recW f (aw a))
      bcw
      (b c -> recW f (bcw b c))
      d
      w
      (recW f w)

data N = Zero | Succ N

foldN :: x -> (x -> x) -> N -> x
foldN z s Zero = z
foldN z s (Succ n) = s (foldN z s n)

recN :: x -> (N -> x -> x) -> N -> x
recN z s Zero = z
recN z s (Succ n) = s n (recN z s n)

recN :: x -> (N -> x -> x) -> N -> x
recN z s n = snd (foldN (Zero, z) ((m, x) -> (Succ m, s m x)) n)

recW :: ((A - W) -> (A -> x) -> (B -> C -> W) -> (B -> C -> x) -> D -> W -> x -> x)
     -> W
     -> x
recW f w =
    snd (foldW
          (awx bcwx d (w', x) ->
              (Constr (a -> fst (awx a)) (b c -> fst (bcwx b c)) d w',
               f (a -> fst (awx a))
                 (a -> snd (awx a))
                 (b c -> fst (bcwx b c))
                 (b c -> snd (bcwx b c))
                 d
                 w'
                 x))
          w)