template <typename Attribute, typename ActualAttribute>
    struct make_attribute
    {
        typedef typename remove_const<Attribute>::type attribute_type;
        typedef typename
            mpl::if_<
                is_same<typename remove_const<ActualAttribute>::type, unused_type>
              , attribute_type
              , ActualAttribute&>::type
        type;

        typedef typename
            mpl::if_<
                is_same<typename remove_const<ActualAttribute>::type, unused_type>
              , attribute_type
              , ActualAttribute>::type
        value_type;

        static Attribute call(unused_type)
        {
             // synthesize the attribute/parameter
            return boost::get(value_initialized<attribute_type>());
        }

        template <typename T>
        static T& call(T& value)
        {
            return value; // just pass the one provided
        }
    };