code: def _gen_toc(self, idx):
          
          if not self._chapterization:
              return ''
  
          start, end = self._start_end_of_index(idx)
          chap_infos = '\n'.join(["            <li><a href=\"#ch{}\">{}</a></li>".format(
              chap._idx,
              chap._title) for chap in self._chapters[start:end]])
          toc = """
      <div id="toc">
          <h2>
              目录<br />
          </h2>
          <ul>
  {}
          </ul>
      </div>
      <div class="pagebreak"></div>
          """ .format(chap_infos)
          return toc
docs: 生成目录html
          :return:
          :rtype: