// --------------------------------------------------------------------------
    // identity
    // --------------------------------------------------------------------------
    template <class Arg>
    struct identity : std::unary_function<Arg,Arg>
    {
        Arg operator()(typename call_traits<Arg>::param_type x)
        {
            return x;
        }
    };  

    // --------------------------------------------------------------------------
    // project1st, project2nd
    // --------------------------------------------------------------------------
    template <class Arg1, class Arg2>
    struct project1st : std::binary_function<Arg1,Arg2,Arg1>
    {
        Arg1 operator()(typename call_traits<Arg1>::param_type x,
                        typename call_traits<Arg2>::param_type y)
        {               
            return x;
        }
    };  

    template <class Arg1, class Arg2>
    struct project2nd : std::binary_function<Arg1,Arg2,Arg2>
    {
        Arg2 operator()(typename call_traits<Arg1>::param_type x,
                        typename call_traits<Arg2>::param_type y)
        {               
            return y;
        }
    };  

    // --------------------------------------------------------------------------
    // select1st, select2nd
    // --------------------------------------------------------------------------
    template <class Pair>
    struct select1st : std::unary_function<Pair,typename Pair::first_type>
    {
        typename Pair::first_type operator()(typename call_traits<Pair>::param_type x)
        {
            return x.first;
        }
    };

    template <class Pair>
    struct select2nd : std::unary_function<Pair,typename Pair::second_type>
    {
        typename Pair::second_type operator()(typename call_traits<Pair>::param_type x)
        {
            return x.second;
        }
    };