--- pop

    ǤƤ֤ޤλ
    nil ֤ޤ

    push, shift, unshift ⻲Ȥ
    Ƥ

    :
          array = [1, [2, 3], 4]
          p array.pop      # => 4
          p array.pop      # => [2, 3]
          p array          # => [1]

          p array.pop      # => 1
          p array.pop      # => nil
          p array          # => []

