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

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

(* 目的：木の中の整数の合計を返す *)
(* sum_tree : tree_t -> int *)
let rec sum_tree tree = match tree with
    Empty -> 0
  | Node (l, n, r) -> sum_tree l + n + sum_tree r

(* テスト *)
let test0 = sum_tree tree0 = 0
let test1 = sum_tree tree1 = 3
let test2 = sum_tree tree2 = 10
let test3 = sum_tree tree3 = 24
let test4 = sum_tree tree4 = 51