docs: def _prior_headerfooter(self):
          """|_Footer| proxy on prior sectPr element or None if this is first section."""
          preceding_sectPr = self._sectPr.preceding_sectPr
          return (
              None if preceding_sectPr is None
              else _Footer(preceding_sectPr, self._document_part, self._hdrftr_index)
          )