(* 目的：受け取ったリストの中から偶数のみを抜き出したリストを返す *) 
(* even : int list -> int list *) 
let rec even lst = match lst with 
    [] -> [] 
  | first :: rest -> 
      if first mod 2 = 0 
      then first :: even rest 
      else even rest 
 
(* テスト *) 
let test1 = even [] = [] 
let test2 = even [3] = [] 
let test3 = even [2] = [2] 
let test4 = even [2; 1; 6; 4; 7] = [2; 6; 4] 
let test5 = even [1; 2; 6; 4; 7] = [2; 6; 4]