What is ScatterHierarchyTag means?

[Nobody/Anonymous]

I read the Loki source code, I doubt why use ScatterHierarchyTag head?

---------------------------------------------------------------------------­-------------------------------------

namespace Private
    {

        template<class, class>
        struct ScatterHierarchyTag;
    }

    template <class TList, template <class> class Unit>
    class GenScatterHierarchy;

    template <class T1, class T2, template <class> class Unit>
    class GenScatterHierarchy<Typelist<T1, T2>, Unit>
        : public GenScatterHierarchy<Private::ScatterHierarchyTag<T1,
T2>, Unit>
        , public GenScatterHierarchy<T2, Unit>
    {
    public:
        typedef Typelist<T1, T2> TList;
        typedef GenScatterHierarchy<Private::ScatterHierarchyTag<T1,
T2>, Unit> LeftBase;
        typedef GenScatterHierarchy<T2, Unit> RightBase;
        template <typename T> struct Rebind
        {
            typedef Unit<T> Result;
        };
    };

    template <class T1, class T2, template <class> class Unit>
    class GenScatterHierarchy<Private::ScatterHierarchyTag<T1, T2>,
Unit>
        : public GenScatterHierarchy<T1, Unit>
    {
    };

    template <class AtomicType, template <class> class Unit>
    class GenScatterHierarchy : public Unit<AtomicType>
    {
        typedef Unit<AtomicType> LeftBase;
        template <typename T> struct Rebind
        {
            typedef Unit<T> Result;
        };
    };

    template <template <class> class Unit>
    class GenScatterHierarchy<NullType, Unit>
    {
        template <typename T> struct Rebind
        {
            typedef Unit<T> Result;
        };

[Fraser]

It ensures that multiple base classes are not identical and ambiguous to a derived class.