template<int __i, int... _Indexes, typename _IdxHolder, typename...
         _Elements>
struct __index_holder_impl<__i, __index_holder<_Indexes...>,
                           _IdxHolder, _Elements...>
{
   typedef typename __index_holder_impl<__i + 1,
                                        __index_holder<_Indexes...,
                                                       __i>,
                                        _Elements...>::type type;
};