type nat = Zero | S of nat
;;

type 'a list = Nil | Cons of 'a * 'a list
;;

let rec foldr f z xs =
  match xs with
  | Nil -> z
  | Cons(x,xs') -> f x (foldr f z xs')
;;

let rec plus x y =
  match x with
  | Zero -> y
  | S(x') -> S(plus x' y)
;;

let rec map f xs =
  match xs with
  | Nil -> Nil
  | Cons(x,xs') -> Cons(f x, map f xs')
;;

let comp f g z = f (g z)
;;

let id x = x
;;

let foldsum l = foldr comp id (map plus l) Zero
;;
()