code: def footnote_ref(self, key, index):
          
          html = (
              '<sup class="footnote-ref" id="fnref-%s">'
              '<a href="#fn-%s">%d</a></sup>'
          ) % (escape(key), escape(key), index)
          return html
docs: Rendering the ref anchor of a footnote.
  
          :param key: identity key for the footnote.
          :param index: the index count of current footnote.