docs: def _tr_below(self):
          """
          The tr element next in sequence after the tr this cell appears in, or
          |None| if this cell appears in the last row.
          """
          tr_lst = self._tbl.tr_lst
          tr_idx = tr_lst.index(self._tr)
          try:
              return tr_lst[tr_idx+1]
          except IndexError:
              return None