constexpr bool is_most_deeply_nested_level(Level const l) noexcept
constexpr auto level_up(Level const l) noexcept
constexpr bool is_above_most_deeply_nested_level(Level const l) noexcept
constexpr bool is_level_contained_by(Level const l1, Level const l2) noexcept
constexpr bool is_highest_level(Level const l) noexcept
constexpr auto level_down(Level const l) noexcept