#ifndef BOOST_PP_IS_ITERATING
    ///////////////////////////////////////////////////////////////////////////////
    /// \file traits.hpp
    /// Contains definitions for child\<\>, child_c\<\>, left\<\>,
    /// right\<\>, tag_of\<\>, and the helper functions child(), child_c(),
    /// value(), left() and right().
    //
    //  Copyright 2008 Eric Niebler. Distributed under the Boost
    //  Software License, Version 1.0. (See accompanying file
    //  LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)

    #ifndef BOOST_PROTO_ARG_TRAITS_HPP_EAN_04_01_2005
    #define BOOST_PROTO_ARG_TRAITS_HPP_EAN_04_01_2005

    #include <boost/config.hpp>
    #include <boost/detail/workaround.hpp>
    #include <boost/preprocessor/iteration/iterate.hpp>
    #include <boost/preprocessor/repetition/enum.hpp>
    #include <boost/preprocessor/repetition/enum_params.hpp>
    #include <boost/preprocessor/repetition/enum_trailing_params.hpp>
    #include <boost/preprocessor/repetition/repeat.hpp>
    #include <boost/preprocessor/repetition/repeat_from_to.hpp>
    #include <boost/preprocessor/facilities/intercept.hpp>
    #include <boost/preprocessor/arithmetic/sub.hpp>
    #include <boost/static_assert.hpp>
    #include <boost/mpl/bool.hpp>
    #include <boost/mpl/aux_/template_arity.hpp>
    #include <boost/mpl/aux_/lambda_arity_param.hpp>
    #include <boost/type_traits/is_pod.hpp>
    #include <boost/type_traits/is_same.hpp>
    #include <boost/type_traits/add_const.hpp>
    #include <boost/proto/proto_fwd.hpp>
    #include <boost/proto/args.hpp>
    #include <boost/proto/domain.hpp>
    #include <boost/proto/transform/pass_through.hpp>

    #if BOOST_WORKAROUND( BOOST_MSVC, >= 1400 )
        #pragma warning(push)
        #pragma warning(disable: 4180) // warning C4180: qualifier applied to function type has no meaning; ignored
    #endif

    namespace boost { namespace proto
    {
        namespace detail
        {
            template<typename T, typename Void = void>
            struct if_vararg
            {};

            template<typename T>
            struct if_vararg<T, typename T::proto_is_vararg_>
              : T
            {};

            template<typename T, typename Void = void>
            struct is_callable2_
              : mpl::false_
            {};

            template<typename T>
            struct is_callable2_<T, typename T::proto_is_callable_>
              : mpl::true_
            {};

            template<typename T BOOST_MPL_AUX_LAMBDA_ARITY_PARAM(long Arity = mpl::aux::template_arity<T>::value)>
            struct is_callable_
              : is_callable2_<T>
            {};
        }

        /// \brief Boolean metafunction which detects whether a type is
        /// a callable function object type or not.
        ///
        /// <tt>is_callable\<\></tt> is used by the <tt>when\<\></tt> transform
        /// to determine whether a function type <tt>R(A1,A2,...AN)</tt> is a
        /// callable transform or an object transform. (The former are evaluated
        /// using <tt>call\<\></tt> and the later with <tt>make\<\></tt>.) If
        /// <tt>is_callable\<R\>::value</tt> is \c true, the function type is
        /// a callable transform; otherwise, it is an object transform.
        ///
        /// Unless specialized for a type \c T, <tt>is_callable\<T\>::value</tt>
        /// is computed as follows:
        ///
        /// \li If \c T is a template type <tt>X\<Y0,Y1,...YN\></tt>, where all \c Yx
        /// are types for \c x in <tt>[0,N]</tt>, <tt>is_callable\<T\>::value</tt>
        /// is <tt>is_same\<YN, proto::callable\>::value</tt>.
        /// \li If \c T has a nested type \c proto_is_callable_ that is a typedef
        /// for \c void, <tt>is_callable\<T\>::value</tt> is \c true. (Note: this is
        /// the case for any type that derives from \c proto::callable.)
        /// \li Otherwise, <tt>is_callable\<T\>::value</tt> is \c false.
        template<typename T>
        struct is_callable
          : proto::detail::is_callable_<T>
        {};

        /// INTERNAL ONLY
        ///
        template<>
        struct is_callable<proto::_>
          : mpl::true_
        {};

        /// INTERNAL ONLY
        ///
        template<>
        struct is_callable<proto::callable>
          : mpl::false_
        {};

        #if BOOST_WORKAROUND(__GNUC__, == 3) || (__GNUC__ == 4 && __GNUC_MINOR__ == 0)
        // work around GCC bug
        template<typename Tag, typename Args, long N>
        struct is_callable<proto::expr<Tag, Args, N> >
          : mpl::false_
        {};

        // work around GCC bug
        template<typename Tag, typename Args, long N>
        struct is_callable<proto::basic_expr<Tag, Args, N> >
          : mpl::false_
        {};
        #endif

        /// \brief A Boolean metafunction that indicates whether a type requires
        /// aggregate initialization.
        ///
        /// <tt>is_aggregate\<\></tt> is used by the <tt>make\<\></tt> transform
        /// to determine how to construct an object of some type \c T, given some
        /// initialization arguments <tt>a0,a1,...aN</tt>.
        /// If <tt>is_aggregate\<T\>::value</tt> is \c true, then an object of
        /// type T will be initialized as <tt>T t = {a0,a1,...aN};</tt>. Otherwise,
        /// it will be initialized as <tt>T t(a0,a1,...aN)</tt>.
        template<typename T, typename Void>
        struct is_aggregate
          : is_pod<T>
        {};

        /// \brief Specialization of <tt>is_aggregate\<\></tt> that indicates
        /// that objects of <tt>expr\<\></tt> type require aggregate initialization.
        template<typename Tag, typename Args, long N>
        struct is_aggregate<proto::expr<Tag, Args, N>, void>
          : mpl::true_
        {};

        template<typename Tag, typename Args, long N>
        struct is_aggregate<proto::basic_expr<Tag, Args, N>, void>
          : mpl::true_
        {};

        /// INTERNAL ONLY
        template<typename T>
        struct is_aggregate<T, typename T::proto_is_aggregate_>
          : mpl::true_
        {};

        /// TODO document me!
        template<typename T, typename Void /* = void*/>
        struct is_transform
          : mpl::false_
        {};

        template<typename T>
        struct is_transform<T, typename T::proto_is_transform_>
          : mpl::true_
        {};

        /// \brief A Boolean metafunction that indicates whether a given
        /// type \c T is a Proto expression type.
        ///
        /// If \c T has a nested type \c proto_is_expr_ that is a typedef
        /// for \c void, <tt>is_expr\<T\>::value</tt> is \c true. (Note, this
        /// is the case for <tt>proto::expr\<\></tt>, any type that is derived
        /// from <tt>proto::extends\<\></tt> or that uses the
        /// <tt>BOOST_PROTO_BASIC_EXTENDS()</tt> macro.) Otherwise,
        /// <tt>is_expr\<T\>::value</tt> is \c false.
        template<typename T, typename Void /* = void*/>
        struct is_expr
          : mpl::false_
        {};

        /// \brief A Boolean metafunction that indicates whether a given
        /// type \c T is a Proto expression type.
        ///
        /// If \c T has a nested type \c proto_is_expr_ that is a typedef
        /// for \c void, <tt>is_expr\<T\>::value</tt> is \c true. (Note, this
        /// is the case for <tt>proto::expr\<\></tt>, any type that is derived
        /// from <tt>proto::extends\<\></tt> or that uses the
        /// <tt>BOOST_PROTO_BASIC_EXTENDS()</tt> macro.) Otherwise,
        /// <tt>is_expr\<T\>::value</tt> is \c false.
        template<typename T>
        struct is_expr<T, typename T::proto_is_expr_>
          : mpl::true_
        {};
            
        template<typename T>
        struct is_expr<T &, void>
          : is_expr<T>
        {};

        /// \brief A metafunction that returns the tag type of a
        /// Proto expression.
        template<typename Expr>
        struct tag_of
        {
            typedef typename Expr::proto_tag type;
        };

        template<typename Expr>
        struct tag_of<Expr &>
        {
            typedef typename Expr::proto_tag type;
        };

        /// \brief A metafunction that returns the arity of a
        /// Proto expression.
        template<typename Expr>
        struct arity_of
          : Expr::proto_arity
        {};

        template<typename Expr>
        struct arity_of<Expr &>
          : Expr::proto_arity
        {};

        namespace result_of
        {
            /// \brief A metafunction that computes the return type of the \c as_expr()
            /// function.
            template<typename T, typename Domain /*= default_domain*/>
            struct as_expr
            {
                typedef typename Domain::template as_expr<T>::result_type type;
            };

            /// \brief A metafunction that computes the return type of the \c as_child()
            /// function.
            template<typename T, typename Domain /*= default_domain*/>
            struct as_child
            {
                typedef typename Domain::template as_child<T>::result_type type;
            };

            /// \brief A metafunction that returns the type of the Nth child
            /// of a Proto expression, where N is an MPL Integral Constant.
            ///
            /// <tt>result_of::child\<Expr, N\></tt> is equivalent to
            /// <tt>result_of::child_c\<Expr, N::value\></tt>.
            template<typename Expr, typename N /* = mpl::long_<0>*/>
            struct child
              : child_c<Expr, N::value>
            {};

            /// \brief A metafunction that returns the type of the value
            /// of a terminal Proto expression.
            ///
            template<typename Expr>
            struct value
            {
                /// Verify that we are actually operating on a terminal
                BOOST_STATIC_ASSERT(0 == Expr::proto_arity_c);

                /// The raw type of the Nth child as it is stored within
                /// \c Expr. This may be a value or a reference
                typedef typename Expr::proto_child0 value_type;

                /// The "value" type of the child, suitable for storage by value,
                /// computed as follows:
                /// \li <tt>T const(&)[N]</tt> becomes <tt>T[N]</tt>
                /// \li <tt>T[N]</tt> becomes <tt>T[N]</tt>
                /// \li <tt>T(&)[N]</tt> becomes <tt>T[N]</tt>
                /// \li <tt>R(&)(A0,...)</tt> becomes <tt>R(&)(A0,...)</tt>
                /// \li <tt>T const &</tt> becomes <tt>T</tt>
                /// \li <tt>T &</tt> becomes <tt>T</tt>
                /// \li <tt>T</tt> becomes <tt>T</tt>
                typedef typename detail::term_traits<typename Expr::proto_child0>::value_type type;
            };

            template<typename Expr>
            struct value<Expr &>
            {
                /// Verify that we are actually operating on a terminal
                BOOST_STATIC_ASSERT(0 == Expr::proto_arity_c);

                /// The raw type of the Nth child as it is stored within
                /// \c Expr. This may be a value or a reference
                typedef typename Expr::proto_child0 value_type;

                /// The "reference" type of the child, suitable for storage by
                /// reference, computed as follows:
                /// \li <tt>T const(&)[N]</tt> becomes <tt>T const(&)[N]</tt>
                /// \li <tt>T[N]</tt> becomes <tt>T(&)[N]</tt>
                /// \li <tt>T(&)[N]</tt> becomes <tt>T(&)[N]</tt>
                /// \li <tt>R(&)(A0,...)</tt> becomes <tt>R(&)(A0,...)</tt>
                /// \li <tt>T const &</tt> becomes <tt>T const &</tt>
                /// \li <tt>T &</tt> becomes <tt>T &</tt>
                /// \li <tt>T</tt> becomes <tt>T &</tt>
                typedef typename detail::term_traits<typename Expr::proto_child0>::reference type;
            };

            template<typename Expr>
            struct value<Expr const &>
            {
                /// Verify that we are actually operating on a terminal
                BOOST_STATIC_ASSERT(0 == Expr::proto_arity_c);

                /// The raw type of the Nth child as it is stored within
                /// \c Expr. This may be a value or a reference
                typedef typename Expr::proto_child0 value_type;

                /// The "const reference" type of the child, suitable for storage by
                /// const reference, computed as follows:
                /// \li <tt>T const(&)[N]</tt> becomes <tt>T const(&)[N]</tt>
                /// \li <tt>T[N]</tt> becomes <tt>T const(&)[N]</tt>
                /// \li <tt>T(&)[N]</tt> becomes <tt>T(&)[N]</tt>
                /// \li <tt>R(&)(A0,...)</tt> becomes <tt>R(&)(A0,...)</tt>
                /// \li <tt>T const &</tt> becomes <tt>T const &</tt>
                /// \li <tt>T &</tt> becomes <tt>T &</tt>
                /// \li <tt>T</tt> becomes <tt>T const &</tt>
                typedef typename detail::term_traits<typename Expr::proto_child0>::const_reference type;
            };

            /// \brief A metafunction that returns the type of the left child
            /// of a binary Proto expression.
            ///
            /// <tt>result_of::left\<Expr\></tt> is equivalent to
            /// <tt>result_of::child_c\<Expr, 0\></tt>.
            template<typename Expr>
            struct left
              : child_c<Expr, 0>
            {};

            /// \brief A metafunction that returns the type of the right child
            /// of a binary Proto expression.
            ///
            /// <tt>result_of::right\<Expr\></tt> is equivalent to
            /// <tt>result_of::child_c\<Expr, 1\></tt>.
            template<typename Expr>
            struct right
              : child_c<Expr, 1>
            {};

        } // namespace result_of

        /// \brief A metafunction for generating terminal expression types,
        /// a grammar element for matching terminal expressions, and a
        /// PrimitiveTransform that returns the current expression unchanged.
        template<typename T>
        struct terminal
          : proto::transform<terminal<T>, int>
        {
            typedef proto::expr<proto::tag::terminal, term<T>, 0> type;
            typedef proto::basic_expr<proto::tag::terminal, term<T>, 0> proto_grammar;

            template<typename Expr, typename State, typename Data>
            struct impl : transform_impl<Expr, State, Data>
            {
                typedef Expr result_type;

                /// \param e The current expression
                /// \pre <tt>matches\<Expr, terminal\<T\> \>::value</tt> is \c true.
                /// \return \c e
                /// \throw nothrow
                #ifdef BOOST_PROTO_STRICT_RESULT_OF
                result_type
                #else
                typename impl::expr_param
                #endif
                operator ()(
                    typename impl::expr_param e
                  , typename impl::state_param
                  , typename impl::data_param
                ) const
                {
                    return e;
                }
            };

            /// INTERNAL ONLY
            typedef proto::tag::terminal proto_tag;
            /// INTERNAL ONLY
            typedef T proto_child0;
        };

        /// \brief A metafunction for generating ternary conditional expression types,
        /// a grammar element for matching ternary conditional expressions, and a
        /// PrimitiveTransform that dispatches to the <tt>pass_through\<\></tt>
        /// transform.
        template<typename T, typename U, typename V>
        struct if_else_
          : proto::transform<if_else_<T, U, V>, int>
        {
            typedef proto::expr<proto::tag::if_else_, list3<T, U, V>, 3> type;
            typedef proto::basic_expr<proto::tag::if_else_, list3<T, U, V>, 3> proto_grammar;

            template<typename Expr, typename State, typename Data>
            struct impl
              : detail::pass_through_impl<if_else_, Expr, State, Data>
            {};

            /// INTERNAL ONLY
            typedef proto::tag::if_else_ proto_tag;
            /// INTERNAL ONLY
            typedef T proto_child0;
            /// INTERNAL ONLY
            typedef U proto_child1;
            /// INTERNAL ONLY
            typedef V proto_child2;
        };

        /// \brief A metafunction for generating nullary expression types with a
        /// specified tag type,
        /// a grammar element for matching nullary expressions, and a
        /// PrimitiveTransform that returns the current expression unchanged.
        ///
        /// Use <tt>nullary_expr\<_, _\></tt> as a grammar element to match any
        /// nullary expression.
        template<typename Tag, typename T>
        struct nullary_expr
          : proto::transform<nullary_expr<Tag, T>, int>
        {
            typedef proto::expr<Tag, term<T>, 0> type;
            typedef proto::basic_expr<Tag, term<T>, 0> proto_grammar;

            template<typename Expr, typename State, typename Data>
            struct impl : transform_impl<Expr, State, Data>
            {
                typedef Expr result_type;

                /// \param e The current expression
                /// \pre <tt>matches\<Expr, nullary_expr\<Tag, T\> \>::value</tt> is \c true.
                /// \return \c e
                /// \throw nothrow
                #ifdef BOOST_PROTO_STRICT_RESULT_OF
                result_type
                #else
                typename impl::expr_param
                #endif
                operator ()(
                    typename impl::expr_param e
                  , typename impl::state_param
                  , typename impl::data_param
                ) const
                {
                    return e;
                }
            };

            /// INTERNAL ONLY
            typedef Tag proto_tag;
            /// INTERNAL ONLY
            typedef T proto_child0;
        };

        /// \brief A metafunction for generating unary expression types with a
        /// specified tag type,
        /// a grammar element for matching unary expressions, and a
        /// PrimitiveTransform that dispatches to the <tt>pass_through\<\></tt>
        /// transform.
        ///
        /// Use <tt>unary_expr\<_, _\></tt> as a grammar element to match any
        /// unary expression.
        template<typename Tag, typename T>
        struct unary_expr
          : proto::transform<unary_expr<Tag, T>, int>
        {
            typedef proto::expr<Tag, list1<T>, 1> type;
            typedef proto::basic_expr<Tag, list1<T>, 1> proto_grammar;

            template<typename Expr, typename State, typename Data>
            struct impl
              : detail::pass_through_impl<unary_expr, Expr, State, Data>
            {};

            /// INTERNAL ONLY
            typedef Tag proto_tag;
            /// INTERNAL ONLY
            typedef T proto_child0;
        };

        /// \brief A metafunction for generating binary expression types with a
        /// specified tag type,
        /// a grammar element for matching binary expressions, and a
        /// PrimitiveTransform that dispatches to the <tt>pass_through\<\></tt>
        /// transform.
        ///
        /// Use <tt>binary_expr\<_, _, _\></tt> as a grammar element to match any
        /// binary expression.
        template<typename Tag, typename T, typename U>
        struct binary_expr
          : proto::transform<binary_expr<Tag, T, U>, int>
        {
            typedef proto::expr<Tag, list2<T, U>, 2> type;
            typedef proto::basic_expr<Tag, list2<T, U>, 2> proto_grammar;

            template<typename Expr, typename State, typename Data>
            struct impl
              : detail::pass_through_impl<binary_expr, Expr, State, Data>
            {};

            /// INTERNAL ONLY
            typedef Tag proto_tag;
            /// INTERNAL ONLY
            typedef T proto_child0;
            /// INTERNAL ONLY
            typedef U proto_child1;
        };

    #define BOOST_PROTO_DEFINE_UNARY_METAFUNCTION(Op)                                               \
        template<typename T>                                                                        \
        struct Op                                                                                   \
          : proto::transform<Op<T>, int>                                                            \
        {                                                                                           \
            typedef proto::expr<proto::tag::Op, list1<T>, 1> type;                                  \
            typedef proto::basic_expr<proto::tag::Op, list1<T>, 1> proto_grammar;                   \
                                                                                                    \
            template<typename Expr, typename State, typename Data>                                  \
            struct impl                                                                             \
              : detail::pass_through_impl<Op, Expr, State, Data>                                    \
            {};                                                                                     \
                                                                                                    \
            typedef proto::tag::Op proto_tag;                                                       \
            typedef T proto_child0;                                                                 \
        };                                                                                          \
        /**/

    #define BOOST_PROTO_DEFINE_BINARY_METAFUNCTION(Op)                                              \
        template<typename T, typename U>                                                            \
        struct Op                                                                                   \
          : proto::transform<Op<T, U>, int>                                                         \
        {                                                                                           \
            typedef proto::expr<proto::tag::Op, list2<T, U>, 2> type;                               \
            typedef proto::basic_expr<proto::tag::Op, list2<T, U>, 2> proto_grammar;                \
                                                                                                    \
            template<typename Expr, typename State, typename Data>                                  \
            struct impl                                                                             \
              : detail::pass_through_impl<Op, Expr, State, Data>                                    \
            {};                                                                                     \
                                                                                                    \
            typedef proto::tag::Op proto_tag;                                                       \
            typedef T proto_child0;                                                                 \
            typedef U proto_child1;                                                                 \
        };                                                                                          \
        /**/

        BOOST_PROTO_DEFINE_UNARY_METAFUNCTION(unary_plus)
        BOOST_PROTO_DEFINE_UNARY_METAFUNCTION(negate)
        BOOST_PROTO_DEFINE_UNARY_METAFUNCTION(dereference)
        BOOST_PROTO_DEFINE_UNARY_METAFUNCTION(complement)
        BOOST_PROTO_DEFINE_UNARY_METAFUNCTION(address_of)
        BOOST_PROTO_DEFINE_UNARY_METAFUNCTION(logical_not)
        BOOST_PROTO_DEFINE_UNARY_METAFUNCTION(pre_inc)
        BOOST_PROTO_DEFINE_UNARY_METAFUNCTION(pre_dec)
        BOOST_PROTO_DEFINE_UNARY_METAFUNCTION(post_inc)
        BOOST_PROTO_DEFINE_UNARY_METAFUNCTION(post_dec)

        BOOST_PROTO_DEFINE_BINARY_METAFUNCTION(shift_left)
        BOOST_PROTO_DEFINE_BINARY_METAFUNCTION(shift_right)
        BOOST_PROTO_DEFINE_BINARY_METAFUNCTION(multiplies)
        BOOST_PROTO_DEFINE_BINARY_METAFUNCTION(divides)
        BOOST_PROTO_DEFINE_BINARY_METAFUNCTION(modulus)
        BOOST_PROTO_DEFINE_BINARY_METAFUNCTION(plus)
        BOOST_PROTO_DEFINE_BINARY_METAFUNCTION(minus)
        BOOST_PROTO_DEFINE_BINARY_METAFUNCTION(less)
        BOOST_PROTO_DEFINE_BINARY_METAFUNCTION(greater)
        BOOST_PROTO_DEFINE_BINARY_METAFUNCTION(less_equal)
        BOOST_PROTO_DEFINE_BINARY_METAFUNCTION(greater_equal)
        BOOST_PROTO_DEFINE_BINARY_METAFUNCTION(equal_to)
        BOOST_PROTO_DEFINE_BINARY_METAFUNCTION(not_equal_to)
        BOOST_PROTO_DEFINE_BINARY_METAFUNCTION(logical_or)
        BOOST_PROTO_DEFINE_BINARY_METAFUNCTION(logical_and)
        BOOST_PROTO_DEFINE_BINARY_METAFUNCTION(bitwise_or)
        BOOST_PROTO_DEFINE_BINARY_METAFUNCTION(bitwise_and)
        BOOST_PROTO_DEFINE_BINARY_METAFUNCTION(bitwise_xor)
        BOOST_PROTO_DEFINE_BINARY_METAFUNCTION(comma)
        BOOST_PROTO_DEFINE_BINARY_METAFUNCTION(mem_ptr)
        BOOST_PROTO_DEFINE_BINARY_METAFUNCTION(assign)
        BOOST_PROTO_DEFINE_BINARY_METAFUNCTION(shift_left_assign)
        BOOST_PROTO_DEFINE_BINARY_METAFUNCTION(shift_right_assign)
        BOOST_PROTO_DEFINE_BINARY_METAFUNCTION(multiplies_assign)
        BOOST_PROTO_DEFINE_BINARY_METAFUNCTION(divides_assign)
        BOOST_PROTO_DEFINE_BINARY_METAFUNCTION(modulus_assign)
        BOOST_PROTO_DEFINE_BINARY_METAFUNCTION(plus_assign)
        BOOST_PROTO_DEFINE_BINARY_METAFUNCTION(minus_assign)
        BOOST_PROTO_DEFINE_BINARY_METAFUNCTION(bitwise_or_assign)
        BOOST_PROTO_DEFINE_BINARY_METAFUNCTION(bitwise_and_assign)
        BOOST_PROTO_DEFINE_BINARY_METAFUNCTION(bitwise_xor_assign)
        BOOST_PROTO_DEFINE_BINARY_METAFUNCTION(subscript)
        BOOST_PROTO_DEFINE_BINARY_METAFUNCTION(member)

    #undef BOOST_PROTO_DEFINE_UNARY_METAFUNCTION
    #undef BOOST_PROTO_DEFINE_BINARY_METAFUNCTION

    #define BOOST_PROTO_CHILD(Z, N, DATA)                                                           \
        /** INTERNAL ONLY */                                                                        \
        typedef BOOST_PP_CAT(DATA, N) BOOST_PP_CAT(proto_child, N);                                 \
        /**/

    #define BOOST_PP_ITERATION_PARAMS_1 (3, (0, BOOST_PROTO_MAX_ARITY, <boost/proto/traits.hpp>))
    #include BOOST_PP_ITERATE()

    #undef BOOST_PROTO_CHILD
    #undef BOOST_PROTO_IMPLICIT_ARG

        namespace functional
        {
            /// \brief A callable PolymorphicFunctionObject that is
            /// equivalent to the \c as_expr() function.
            template<typename Domain   /* = default_domain*/>
            struct as_expr
            {
                BOOST_PROTO_CALLABLE()

                template<typename Sig>
                struct result;

                template<typename This, typename T>
                struct result<This(T)>
                {
                    typedef typename Domain::template as_expr<T>::result_type type;
                };

                template<typename This, typename T>
                struct result<This(T &)>
                {
                    typedef typename Domain::template as_expr<T>::result_type type;
                };

                /// \brief Wrap an object in a Proto terminal if it isn't a
                /// Proto expression already.
                /// \param t The object to wrap.
                /// \return <tt>proto::as_expr\<Domain\>(t)</tt>
                template<typename T>
                typename add_const<typename result<as_expr(T &)>::type>::type
                operator ()(T &t) const
                {
                    return typename Domain::template as_expr<T>()(t);
                }

                /// \overload
                ///
                template<typename T>
                typename add_const<typename result<as_expr(T const &)>::type>::type
                operator ()(T const &t) const
                {
                    return typename Domain::template as_expr<T const>()(t);
                }

                #if BOOST_WORKAROUND(BOOST_MSVC, == 1310)
                template<typename T, std::size_t N_>
                typename add_const<typename result<as_expr(T (&)[N_])>::type>::type
                operator ()(T (&t)[N_]) const
                {
                    return typename Domain::template as_expr<T[N_]>()(t);
                }

                template<typename T, std::size_t N_>
                typename add_const<typename result<as_expr(T const (&)[N_])>::type>::type
                operator ()(T const (&t)[N_]) const
                {
                    return typename Domain::template as_expr<T const[N_]>()(t);
                }
                #endif
            };

            /// \brief A callable PolymorphicFunctionObject that is
            /// equivalent to the \c as_child() function.
            template<typename Domain   /* = default_domain*/>
            struct as_child
            {
                BOOST_PROTO_CALLABLE()

                template<typename Sig>
                struct result;

                template<typename This, typename T>
                struct result<This(T)>
                {
                    typedef typename Domain::template as_child<T>::result_type type;
                };

                template<typename This, typename T>
                struct result<This(T &)>
                {
                    typedef typename Domain::template as_child<T>::result_type type;
                };

                /// \brief Wrap an object in a Proto terminal if it isn't a
                /// Proto expression already.
                /// \param t The object to wrap.
                /// \return <tt>proto::as_child\<Domain\>(t)</tt>
                template<typename T>
                typename add_const<typename result<as_child(T &)>::type>::type
                operator ()(T &t) const
                {
                    return typename Domain::template as_child<T>()(t);
                }

                /// \overload
                ///
                template<typename T>
                typename add_const<typename result<as_child(T const &)>::type>::type
                operator ()(T const &t) const
                {
                    return typename Domain::template as_child<T const>()(t);
                }
            };

            /// \brief A callable PolymorphicFunctionObject that is
            /// equivalent to the \c child_c() function.
            template<long N>
            struct child_c
            {
                BOOST_PROTO_CALLABLE()

                template<typename Sig>
                struct result;

                template<typename This, typename Expr>
                struct result<This(Expr)>
                {
                    typedef typename result_of::child_c<Expr, N>::type type;
                };

                /// \brief Return the Nth child of the given expression.
                /// \param expr The expression node.
                /// \pre <tt>is_expr\<Expr\>::value</tt> is \c true
                /// \pre <tt>N \< Expr::proto_arity::value</tt>
                /// \return <tt>proto::child_c\<N\>(expr)</tt>
                /// \throw nothrow
                template<typename Expr>
                typename result_of::child_c<Expr &, N>::type
                operator ()(Expr &e) const
                {
                    return result_of::child_c<Expr &, N>::call(e);
                }

                /// \overload
                ///
                template<typename Expr>
                typename result_of::child_c<Expr const &, N>::type
                operator ()(Expr const &e) const
                {
                    return result_of::child_c<Expr const &, N>::call(e);
                }
            };

            /// \brief A callable PolymorphicFunctionObject that is
            /// equivalent to the \c child() function.
            ///
            /// A callable PolymorphicFunctionObject that is
            /// equivalent to the \c child() function. \c N is required
            /// to be an MPL Integral Constant.
            template<typename N /* = mpl::long_<0>*/>
            struct child
            {
                BOOST_PROTO_CALLABLE()

                template<typename Sig>
                struct result;

                template<typename This, typename Expr>
                struct result<This(Expr)>
                {
                    typedef typename result_of::child<Expr, N>::type type;
                };

                /// \brief Return the Nth child of the given expression.
                /// \param expr The expression node.
                /// \pre <tt>is_expr\<Expr\>::value</tt> is \c true
                /// \pre <tt>N::value \< Expr::proto_arity::value</tt>
                /// \return <tt>proto::child\<N\>(expr)</tt>
                /// \throw nothrow
                template<typename Expr>
                typename result_of::child<Expr &, N>::type
                operator ()(Expr &e) const
                {
                    return result_of::child<Expr &, N>::call(e);
                }

                /// \overload
                ///
                template<typename Expr>
                typename result_of::child<Expr const &, N>::type
                operator ()(Expr const &e) const
                {
                    return result_of::child<Expr const &, N>::call(e);
                }
            };

            /// \brief A callable PolymorphicFunctionObject that is
            /// equivalent to the \c value() function.
            struct value
            {
                BOOST_PROTO_CALLABLE()

                template<typename Sig>
                struct result;

                template<typename This, typename Expr>
                struct result<This(Expr)>
                {
                    typedef typename result_of::value<Expr>::type type;
                };

                /// \brief Return the value of the given terminal expression.
                /// \param expr The terminal expression node.
                /// \pre <tt>is_expr\<Expr\>::value</tt> is \c true
                /// \pre <tt>0 == Expr::proto_arity::value</tt>
                /// \return <tt>proto::value(expr)</tt>
                /// \throw nothrow
                template<typename Expr>
                typename result_of::value<Expr &>::type
                operator ()(Expr &e) const
                {
                    return e.proto_base().child0;
                }

                /// \overload
                ///
                template<typename Expr>
                typename result_of::value<Expr const &>::type
                operator ()(Expr const &e) const
                {
                    return e.proto_base().child0;
                }
            };

            /// \brief A callable PolymorphicFunctionObject that is
            /// equivalent to the \c left() function.
            struct left
            {
                BOOST_PROTO_CALLABLE()

                template<typename Sig>
                struct result;

                template<typename This, typename Expr>
                struct result<This(Expr)>
                {
                    typedef typename result_of::left<Expr>::type type;
                };

                /// \brief Return the left child of the given binary expression.
                /// \param expr The expression node.
                /// \pre <tt>is_expr\<Expr\>::value</tt> is \c true
                /// \pre <tt>2 == Expr::proto_arity::value</tt>
                /// \return <tt>proto::left(expr)</tt>
                /// \throw nothrow
                template<typename Expr>
                typename result_of::left<Expr &>::type
                operator ()(Expr &e) const
                {
                    return e.proto_base().child0;
                }

                /// \overload
                ///
                template<typename Expr>
                typename result_of::left<Expr const &>::type
                operator ()(Expr const &e) const
                {
                    return e.proto_base().child0;
                }
            };

            /// \brief A callable PolymorphicFunctionObject that is
            /// equivalent to the \c right() function.
            struct right
            {
                BOOST_PROTO_CALLABLE()

                template<typename Sig>
                struct result;

                template<typename This, typename Expr>
                struct result<This(Expr)>
                {
                    typedef typename result_of::right<Expr>::type type;
                };

                /// \brief Return the right child of the given binary expression.
                /// \param expr The expression node.
                /// \pre <tt>is_expr\<Expr\>::value</tt> is \c true
                /// \pre <tt>2 == Expr::proto_arity::value</tt>
                /// \return <tt>proto::right(expr)</tt>
                /// \throw nothrow
                template<typename Expr>
                typename result_of::right<Expr &>::type
                operator ()(Expr &e) const
                {
                    return e.proto_base().child1;
                }

                template<typename Expr>
                typename result_of::right<Expr const &>::type
                operator ()(Expr const &e) const
                {
                    return e.proto_base().child1;
                }
            };

        }

        /// \brief A function that wraps non-Proto expression types in Proto
        /// terminals and leaves Proto expression types alone.
        ///
        /// The <tt>as_expr()</tt> function turns objects into Proto terminals if
        /// they are not Proto expression types already. Non-Proto types are
        /// held by value, if possible. Types which are already Proto types are
        /// left alone and returned by reference.
        ///
        /// This function can be called either with an explicitly specified
        /// \c Domain parameter (i.e., <tt>as_expr\<Domain\>(t)</tt>), or
        /// without (i.e., <tt>as_expr(t)</tt>). If no domain is
        /// specified, \c default_domain is assumed.
        ///
        /// If <tt>is_expr\<T\>::value</tt> is \c true, then the argument is
        /// returned unmodified, by reference. Otherwise, the argument is wrapped
        /// in a Proto terminal expression node according to the following rules.
        /// If \c T is a function type, let \c A be <tt>T &</tt>. Otherwise, let
        /// \c A be the type \c T stripped of cv-qualifiers. Then, \c as_expr()
        /// returns <tt>Domain()(terminal\<A\>::type::make(t))</tt>.
        ///
        /// \param t The object to wrap.
        template<typename T>
        typename add_const<typename result_of::as_expr<T, default_domain>::type>::type
        as_expr(T &t BOOST_PROTO_DISABLE_IF_IS_CONST(T) BOOST_PROTO_DISABLE_IF_IS_FUNCTION(T))
        {
            return default_domain::as_expr<T>()(t);
        }

        /// \overload
        ///
        template<typename T>
        typename add_const<typename result_of::as_expr<T const, default_domain>::type>::type
        as_expr(T const &t)
        {
            return default_domain::as_expr<T const>()(t);
        }

        /// \overload
        ///
        template<typename Domain, typename T>
        typename add_const<typename result_of::as_expr<T, Domain>::type>::type
        as_expr(T &t BOOST_PROTO_DISABLE_IF_IS_CONST(T) BOOST_PROTO_DISABLE_IF_IS_FUNCTION(T))
        {
            return typename Domain::template as_expr<T>()(t);
        }

        /// \overload
        ///
        template<typename Domain, typename T>
        typename add_const<typename result_of::as_expr<T const, Domain>::type>::type
        as_expr(T const &t)
        {
            return typename Domain::template as_expr<T const>()(t);
        }

        /// \brief A function that wraps non-Proto expression types in Proto
        /// terminals (by reference) and returns Proto expression types by
        /// reference
        ///
        /// The <tt>as_child()</tt> function turns objects into Proto terminals if
        /// they are not Proto expression types already. Non-Proto types are
        /// held by reference. Types which are already Proto types are simply
        /// returned as-is.
        ///
        /// This function can be called either with an explicitly specified
        /// \c Domain parameter (i.e., <tt>as_child\<Domain\>(t)</tt>), or
        /// without (i.e., <tt>as_child(t)</tt>). If no domain is
        /// specified, \c default_domain is assumed.
        ///
        /// If <tt>is_expr\<T\>::value</tt> is \c true, then the argument is
        /// returned as-is. Otherwise, \c as_child() returns
        /// <tt>Domain()(terminal\<T &\>::type::make(t))</tt>.
        ///
        /// \param t The object to wrap.
        template<typename T>
        typename add_const<typename result_of::as_child<T, default_domain>::type>::type
        as_child(T &t BOOST_PROTO_DISABLE_IF_IS_CONST(T) BOOST_PROTO_DISABLE_IF_IS_FUNCTION(T))
        {
            return default_domain::as_child<T>()(t);
        }

        /// \overload
        ///
        template<typename T>
        typename add_const<typename result_of::as_child<T const, default_domain>::type>::type
        as_child(T const &t)
        {
            return default_domain::as_child<T const>()(t);
        }

        /// \overload
        ///
        template<typename Domain, typename T>
        typename add_const<typename result_of::as_child<T, Domain>::type>::type
        as_child(T &t BOOST_PROTO_DISABLE_IF_IS_CONST(T) BOOST_PROTO_DISABLE_IF_IS_FUNCTION(T))
        {
            return typename Domain::template as_child<T>()(t);
        }

        /// \overload
        ///
        template<typename Domain, typename T>
        typename add_const<typename result_of::as_child<T const, Domain>::type>::type
        as_child(T const &t)
        {
            return typename Domain::template as_child<T const>()(t);
        }

        /// \brief Return the Nth child of the specified Proto expression.
        ///
        /// Return the Nth child of the specified Proto expression. If
        /// \c N is not specified, as in \c child(expr), then \c N is assumed
        /// to be <tt>mpl::long_\<0\></tt>. The child is returned by
        /// reference.
        ///
        /// \param expr The Proto expression.
        /// \pre <tt>is_expr\<Expr\>::value</tt> is \c true.
        /// \pre \c N is an MPL Integral Constant.
        /// \pre <tt>N::value \< Expr::proto_arity::value</tt>
        /// \throw nothrow
        /// \return A reference to the Nth child
        template<typename N, typename Expr>
        typename result_of::child<Expr &, N>::type
        child(Expr &e BOOST_PROTO_DISABLE_IF_IS_CONST(Expr))
        {
            return result_of::child<Expr &, N>::call(e);
        }

        /// \overload
        ///
        template<typename N, typename Expr>
        typename result_of::child<Expr const &, N>::type
        child(Expr const &e)
        {
            return result_of::child<Expr const &, N>::call(e);
        }

        /// \overload
        ///
        template<typename Expr2>
        typename detail::expr_traits<typename Expr2::proto_base_expr::proto_child0>::reference
        child(Expr2 &expr2 BOOST_PROTO_DISABLE_IF_IS_CONST(Expr2))
        {
            return expr2.proto_base().child0;
        }

        /// \overload
        ///
        template<typename Expr2>
        typename detail::expr_traits<typename Expr2::proto_base_expr::proto_child0>::const_reference
        child(Expr2 const &expr2)
        {
            return expr2.proto_base().child0;
        }

        /// \brief Return the Nth child of the specified Proto expression.
        ///
        /// Return the Nth child of the specified Proto expression. The child
        /// is returned by reference.
        ///
        /// \param expr The Proto expression.
        /// \pre <tt>is_expr\<Expr\>::value</tt> is \c true.
        /// \pre <tt>N \< Expr::proto_arity::value</tt>
        /// \throw nothrow
        /// \return A reference to the Nth child
        template<long N, typename Expr>
        typename result_of::child_c<Expr &, N>::type
        child_c(Expr &e BOOST_PROTO_DISABLE_IF_IS_CONST(Expr))
        {
            return result_of::child_c<Expr &, N>::call(e);
        }

        /// \overload
        ///
        template<long N, typename Expr>
        typename result_of::child_c<Expr const &, N>::type
        child_c(Expr const &e)
        {
            return result_of::child_c<Expr const &, N>::call(e);
        }

        /// \brief Return the value stored within the specified Proto
        /// terminal expression.
        ///
        /// Return the the value stored within the specified Proto
        /// terminal expression. The value is returned by
        /// reference.
        ///
        /// \param expr The Proto terminal expression.
        /// \pre <tt>N::value == 0</tt>
        /// \throw nothrow
        /// \return A reference to the terminal's value
        template<typename Expr>
        typename result_of::value<Expr &>::type
        value(Expr &e BOOST_PROTO_DISABLE_IF_IS_CONST(Expr))
        {
            return e.proto_base().child0;
        }

        /// \overload
        ///
        template<typename Expr>
        typename result_of::value<Expr const &>::type
        value(Expr const &e)
        {
            return e.proto_base().child0;
        }

        /// \brief Return the left child of the specified binary Proto
        /// expression.
        ///
        /// Return the left child of the specified binary Proto expression. The
        /// child is returned by reference.
        ///
        /// \param expr The Proto expression.
        /// \pre <tt>is_expr\<Expr\>::value</tt> is \c true.
        /// \pre <tt>2 == Expr::proto_arity::value</tt>
        /// \throw nothrow
        /// \return A reference to the left child
        template<typename Expr>
        typename result_of::left<Expr &>::type
        left(Expr &e BOOST_PROTO_DISABLE_IF_IS_CONST(Expr))
        {
            return e.proto_base().child0;
        }

        /// \overload
        ///
        template<typename Expr>
        typename result_of::left<Expr const &>::type
        left(Expr const &e)
        {
            return e.proto_base().child0;
        }

        /// \brief Return the right child of the specified binary Proto
        /// expression.
        ///
        /// Return the right child of the specified binary Proto expression. The
        /// child is returned by reference.
        ///
        /// \param expr The Proto expression.
        /// \pre <tt>is_expr\<Expr\>::value</tt> is \c true.
        /// \pre <tt>2 == Expr::proto_arity::value</tt>
        /// \throw nothrow
        /// \return A reference to the right child
        template<typename Expr>
        typename result_of::right<Expr &>::type
        right(Expr &e BOOST_PROTO_DISABLE_IF_IS_CONST(Expr))
        {
            return e.proto_base().child1;
        }

        /// \overload
        ///
        template<typename Expr>
        typename result_of::right<Expr const &>::type
        right(Expr const &e)
        {
            return e.proto_base().child1;
        }

        /// INTERNAL ONLY
        ///
        template<typename Domain>
        struct is_callable<functional::as_expr<Domain> >
          : mpl::true_
        {};

        /// INTERNAL ONLY
        ///
        template<typename Domain>
        struct is_callable<functional::as_child<Domain> >
          : mpl::true_
        {};

        /// INTERNAL ONLY
        ///
        template<long N>
        struct is_callable<functional::child_c<N> >
          : mpl::true_
        {};

        /// INTERNAL ONLY
        ///
        template<typename N>
        struct is_callable<functional::child<N> >
          : mpl::true_
        {};

    }}

    #if BOOST_WORKAROUND( BOOST_MSVC, >= 1400 )
        #pragma warning(pop)
    #endif

    #endif

#else // PP_IS_ITERATING

    #define N BOOST_PP_ITERATION()
    #if N > 0
        /// \brief A metafunction for generating function-call expression types,
        /// a grammar element for matching function-call expressions, and a
        /// PrimitiveTransform that dispatches to the <tt>pass_through\<\></tt>
        /// transform.
        template<BOOST_PP_ENUM_PARAMS(N, typename A)>
        struct function
        #if N != BOOST_PROTO_MAX_ARITY
        <
            BOOST_PP_ENUM_PARAMS(N, A)
            BOOST_PP_ENUM_TRAILING_PARAMS(BOOST_PP_SUB(BOOST_PROTO_MAX_ARITY, N), void BOOST_PP_INTERCEPT)
        >
        #endif
          : proto::transform<
                function<
                    BOOST_PP_ENUM_PARAMS(N, A)
                    BOOST_PP_ENUM_TRAILING_PARAMS(BOOST_PP_SUB(BOOST_PROTO_MAX_ARITY, N), void BOOST_PP_INTERCEPT)
                >
              , int
            >
        {
            typedef proto::expr<proto::tag::function, BOOST_PP_CAT(list, N)<BOOST_PP_ENUM_PARAMS(N, A)>, N> type;
            typedef proto::basic_expr<proto::tag::function, BOOST_PP_CAT(list, N)<BOOST_PP_ENUM_PARAMS(N, A)>, N> proto_grammar;

            template<typename Expr, typename State, typename Data>
            struct impl
              : detail::pass_through_impl<function, Expr, State, Data>
            {};

            /// INTERNAL ONLY
            typedef proto::tag::function proto_tag;
            BOOST_PP_REPEAT(N, BOOST_PROTO_CHILD, A)
            BOOST_PP_REPEAT_FROM_TO(
                N
              , BOOST_PROTO_MAX_ARITY
              , BOOST_PROTO_CHILD
              , detail::if_vararg<BOOST_PP_CAT(A, BOOST_PP_DEC(N))> BOOST_PP_INTERCEPT
            )
        };

        /// \brief A metafunction for generating n-ary expression types with a
        /// specified tag type,
        /// a grammar element for matching n-ary expressions, and a
        /// PrimitiveTransform that dispatches to the <tt>pass_through\<\></tt>
        /// transform.
        ///
        /// Use <tt>nary_expr\<_, vararg\<_\> \></tt> as a grammar element to match any
        /// n-ary expression; that is, any non-terminal.
        template<typename Tag BOOST_PP_ENUM_TRAILING_PARAMS(N, typename A)>
        struct nary_expr
        #if N != BOOST_PROTO_MAX_ARITY
        <
            Tag
            BOOST_PP_ENUM_TRAILING_PARAMS(N, A)
            BOOST_PP_ENUM_TRAILING_PARAMS(BOOST_PP_SUB(BOOST_PROTO_MAX_ARITY, N), void BOOST_PP_INTERCEPT)
        >
        #endif
          : proto::transform<
                nary_expr<
                    Tag
                    BOOST_PP_ENUM_TRAILING_PARAMS(N, A)
                    BOOST_PP_ENUM_TRAILING_PARAMS(BOOST_PP_SUB(BOOST_PROTO_MAX_ARITY, N), void BOOST_PP_INTERCEPT)
                >
              , int
            >
        {
            typedef proto::expr<Tag, BOOST_PP_CAT(list, N)<BOOST_PP_ENUM_PARAMS(N, A)>, N> type;
            typedef proto::basic_expr<Tag, BOOST_PP_CAT(list, N)<BOOST_PP_ENUM_PARAMS(N, A)>, N> proto_grammar;

            template<typename Expr, typename State, typename Data>
            struct impl
              : detail::pass_through_impl<nary_expr, Expr, State, Data>
            {};

            /// INTERNAL ONLY
            typedef Tag proto_tag;
            BOOST_PP_REPEAT(N, BOOST_PROTO_CHILD, A)
            BOOST_PP_REPEAT_FROM_TO(
                N
              , BOOST_PROTO_MAX_ARITY
              , BOOST_PROTO_CHILD
              , detail::if_vararg<BOOST_PP_CAT(A, BOOST_PP_DEC(N))> BOOST_PP_INTERCEPT
            )
        };

        namespace detail
        {
            template<
                template<BOOST_PP_ENUM_PARAMS(N, typename BOOST_PP_INTERCEPT)> class T
              , BOOST_PP_ENUM_PARAMS(N, typename A)
            >
            struct is_callable_<T<BOOST_PP_ENUM_PARAMS(N, A)> BOOST_MPL_AUX_LAMBDA_ARITY_PARAM(N)>
              : is_same<BOOST_PP_CAT(A, BOOST_PP_DEC(N)), callable>
            {};
        }

    #endif

        namespace result_of
        {
            /// \brief A metafunction that returns the type of the Nth child
            /// of a Proto expression.
            ///
            /// A metafunction that returns the type of the Nth child
            /// of a Proto expression. \c N must be less than
            /// \c Expr::proto_arity::value.
            template<typename Expr>
            struct child_c<Expr, N>
            {
                /// Verify that we are not operating on a terminal
                BOOST_STATIC_ASSERT(0 != Expr::proto_arity_c);

                /// The raw type of the Nth child as it is stored within
                /// \c Expr. This may be a value or a reference
                typedef typename Expr::BOOST_PP_CAT(proto_child, N) value_type;

                /// The "value" type of the child, suitable for return by value,
                /// computed as follows:
                /// \li <tt>T const &</tt> becomes <tt>T</tt>
                /// \li <tt>T &</tt> becomes <tt>T</tt>
                /// \li <tt>T</tt> becomes <tt>T</tt>
                typedef typename detail::expr_traits<typename Expr::BOOST_PP_CAT(proto_child, N)>::value_type type;
            };

            template<typename Expr>
            struct child_c<Expr &, N>
            {
                /// Verify that we are not operating on a terminal
                BOOST_STATIC_ASSERT(0 != Expr::proto_arity_c);

                /// The raw type of the Nth child as it is stored within
                /// \c Expr. This may be a value or a reference
                typedef typename Expr::BOOST_PP_CAT(proto_child, N) value_type;

                /// The "reference" type of the child, suitable for return by
                /// reference, computed as follows:
                /// \li <tt>T const &</tt> becomes <tt>T const &</tt>
                /// \li <tt>T &</tt> becomes <tt>T &</tt>
                /// \li <tt>T</tt> becomes <tt>T &</tt>
                typedef typename detail::expr_traits<typename Expr::BOOST_PP_CAT(proto_child, N)>::reference type;

                /// INTERNAL ONLY
                ///
                static type call(Expr &e)
                {
                    return e.proto_base().BOOST_PP_CAT(child, N);
                }
            };

            template<typename Expr>
            struct child_c<Expr const &, N>
            {
                /// Verify that we are not operating on a terminal
                BOOST_STATIC_ASSERT(0 != Expr::proto_arity_c);

                /// The raw type of the Nth child as it is stored within
                /// \c Expr. This may be a value or a reference
                typedef typename Expr::BOOST_PP_CAT(proto_child, N) value_type;

                /// The "const reference" type of the child, suitable for return by
                /// const reference, computed as follows:
                /// \li <tt>T const &</tt> becomes <tt>T const &</tt>
                /// \li <tt>T &</tt> becomes <tt>T &</tt>
                /// \li <tt>T</tt> becomes <tt>T const &</tt>
                typedef typename detail::expr_traits<typename Expr::BOOST_PP_CAT(proto_child, N)>::const_reference type;

                /// INTERNAL ONLY
                ///
                static type call(Expr const &e)
                {
                    return e.proto_base().BOOST_PP_CAT(child, N);
                }
            };
        }

    #undef N

#endif