--- extend(module ... )

    ǻꤷ⥸塼Ƥ᥽åɤ self 
    ðۥ᥽åɤȤɲäޤ륪֥Ȥ˥⥸塼
    εǽɲäȤ˻Ѥޤ

        module Foo
          def a
            'ok'
          end
        end

        obj = Object.new
        obj.extend Foo
        p obj.a         #=> "ok"

    extend εǽϡðۥ饹Ф include
    ȸؤ뤳ȤǤޤ

    ruby 1.7 feature: ʣΥ⥸塼ꤷ硢Ǹ
    ΰս extend Ԥޤ

