function canBeSubtag ($propName)
  {
    if ($this->defines ($propName)) {
      $type = $this->getTypeOf ($propName);
      switch ($type) {
        case type::content:
        case type::collection:
        case type::metadata:
        case type::string:
        case type::number:
          return true;
      }
    }
    return false;
  }