protected function mergeConfigs(array $old, array $new) : array
    {
      foreach ($new as $key => $val) {
        if (!array_key_exists($key, $old) || !is_array($old[$key]) || !is_array($val)) {
          $old[$key] = $val;
          continue;
        }

        if (is_int($key)) {
          $old[] = $val;
          continue;
        }

        $old[$key] = $this->mergeConfigs($old[$key], $val);
      }

      return $old;
    }