(* ２分木を表す型 *)
type tree_t = Empty
            | Node of tree_t * int * tree_t

(* ２分探索木の例 *)
let tree0 = Empty
let tree1 = Node (Empty,  7, Empty)
let tree2 = Node (Empty,  3, tree1)
let tree3 = Node (Empty, 24, Empty)
let tree4 = Node (tree2, 17, tree3)

(* 目的：木 tree の中に整数 m があるか判定する *)
(* search : tree_t -> int -> bool *)
let rec search tree m = match tree with
    Empty -> false
  | Node (l, n, r) ->
      if m < n then search l m
      else if n < m then search r m
      else (* m = n *) true

(* テスト *)
let test0 = search tree0  0 = false
let test1 = search tree4 17 = true
let test2 = search tree4  7 = true
let test3 = search tree4  5 = false
let test4 = search tree4 20 = false