protected function determineHeaderTags($topLevel, $depth)
    {
        $desired = range((int) $topLevel, (int) $topLevel + ((int) $depth - 1));
        $allowed = [1, 2, 3, 4, 5, 6];

        return array_map(function($val) { return 'h'.$val; }, array_intersect($desired, $allowed));
    }