type nat = Zero | S of nat
;;

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

type bool = False | True
;;

let rec leq x y =
  match x with
    | Zero -> True
    | S(x') ->
       match y with
       | Zero -> False
       | S(y') -> leq x' y'
;;
let rec insert ord x ys =
  match ys with
  | Nil -> Cons(x,Nil)
  | Cons(y,ys') ->
     match ord x y with
     | True -> Cons(x,Cons(y,ys'))
     | False -> Cons(y,insert ord x ys')
;;
let rec fold f z xs =
  match xs with
  | Nil -> z
  | Cons(x,xs') -> f x (fold f z xs')
;;

let isort ys = fold (insert leq) Nil ys
;;
()