--- flatten
--- flatten!

    ͥȤʿ경Ƥ֤ޤflatten! 
    󤽤켫Τ˲Ūʿ경󤬥ͥȤƤʤȤ
    ˤ nil ֤ޤ

    :

        p [1, [2, 3, [4], 5]].flatten   #=> [1, 2, 3, 4, 5]

        array = [[[1, [2, 3]]]]
        array.flatten!
        p array                         #=> [1, 2, 3]

