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

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

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