--- self[nth]

    ¤Τ nth ܤΥФ֤ͤޤnth ʸ
    ޤ Symbol ξϤ̾ΥФ֤ͤޤ

    ¸ߤʤФꤷ硢㳰 IndexError 뤤 
    NameError ȯޤ

        Foo = Struct.new(:foo, :bar)
        obj = Foo.new('FOO', 'BAR')

        p obj[:foo]     # => "FOO"
        p obj['bar']    # => "BAR"
        # p obj[:baz]     # => in `[]': no member 'baz' in struct (NameError)
        p obj[0]        # => "FOO"
        p obj[1]        # => "BAR"
        p obj[-1]       # => "BAR"    # Array Τ褦ΥǥåǤޤ
        p obj[2]        # => in `[]': offset 2 too large for struct(size:2) (IndexError)

