--- Struct::XXX.new(value,...)
--- Struct::XXX[value,...]

    ¤Υ֥Ȥ֤ޤϹ¤ΤνͤǤ
    ¤ΤΥФο¿ꤹȤ 㳰 ArgumentError 
    ȯޤФνͤϻꤵʤ nil Ǥ

           Foo = Struct.new(:foo, :bar)
           foo = Foo.new(1)
           p foo.values      # => [1, nil]

