code: def toc_html(self, depth=6, lowest_level=6):
          
          toc = self.toc(depth=depth, lowest_level=lowest_level)
          if not toc:
              return ''
  
          def map_toc_list(toc_list):
              result = ''
              if toc_list:
                  result += '<ul>\n'
                  result += ''.join(
                      map(lambda x: '<li>'
                                    '<a href="#{}">{}</a>{}'
                                    '</li>\n'.format(
                          x['id'], x['inner_html'],
                          map_toc_list(x['children'])),
                          toc_list)
                  )
                  result += '</ul>'
              return result
  
          return map_toc_list(toc)
docs: Get TOC of currently fed HTML string in form of HTML string.
  
          :param depth: the depth of TOC
          :param lowest_level: the allowed lowest level of header tag
          :return: an HTML string