let rec rev_append l1 l2 =
  match l1 with
    [] -> l2
  | a :: l -> rev_append l (a :: l2)

let rev l = rev_append l []

let find_all p =
  let rec find accu = function
  | [] -> rev accu
  | x :: l -> if p x then find (x :: accu) l else find accu l in
  find []

let filter = find_all

let rec map f = function
    [] -> []
  | a::l -> let r = f a in r :: map f l

let rec iter f = function
    [] -> ()
  | a::l -> f a; iter f l

let rec fold_right f l accu =
  match l with
    [] -> accu
  | a::l -> f a (fold_right f l accu)