Class adjust_combining_groups

include/boost/parser/parser.hpp:3941–3951

        };
        template<int MaxGroupIdx>
        struct adjust_combining_groups
        {
            template<typename T, typename U>
            constexpr auto operator()(T result, U x) const
            {
                if constexpr (U::value <= 0)
                    return hl::append(result, x);
                else
                    return hl::append(result, llong<MaxGroupIdx + U::value>{});
            }
        };
        template<typename Tuple1, typename Tuple2>
        constexpr auto make_combined_combining(Tuple1 lhs, Tuple2 rhs)
        {