The tr element next in sequence after the tr this cell appears in, or
        |None| if this cell appears in the last row.

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