code: def header_id_from_text(self, text, prefix, n):
          
          header_id = _slugify(text)
          if prefix and isinstance(prefix, base_string_type):
              header_id = prefix + '-' + header_id
          if header_id in self._count_from_header_id:
              self._count_from_header_id[header_id] += 1
              header_id += '-%s' % self._count_from_header_id[header_id]
          else:
              self._count_from_header_id[header_id] = 1
          return header_id
docs: Generate a header id attribute value from the given header
          HTML content.
  
          This is only called if the "header-ids" extra is enabled.
          Subclasses may override this for different header ids.
  
          @param text {str} The text of the header tag
          @param prefix {str} The requested prefix for header ids. This is the
              value of the "header-ids" extra key, if any. Otherwise, None.
          @param n {int} The <hN> tag number, i.e. `1` for an <h1> tag.
          @returns {str} The value for the header tag's "id" attribute. Return
              None to not have an id attribute and to exclude this header from
              the TOC (if the "toc" extra is specified).