private function bookmark_tag() {
    if ( ! $this->get_tag() ) {
      return false;
    }

    if ( ! parent::set_bookmark( ++$this->bookmark_counter ) ) {
      $this->last_error = self::ERROR_EXCEEDED_MAX_BOOKMARKS;
      throw new Exception( 'could not allocate bookmark' );
    }

    return "{$this->bookmark_counter}";
  }