(* 目的：鶴の数 tsuru を受け取ったら足の本数を返す *) 
(* tsuru_no_ashi : int -> int *) 
let tsuru_no_ashi tsuru = tsuru * 2 
 
(* テスト *) 
let test1 = tsuru_no_ashi 0 = 0 
let test2 = tsuru_no_ashi 1 = 2 
let test3 = tsuru_no_ashi 7 = 14