def _tr_above(self):
        """
        The tr element prior in sequence to the tr this cell appears in.
        Raises |ValueError| if called on a cell in the top-most row.
        """
        tr_lst = self._tbl.tr_lst
        tr_idx = tr_lst.index(self._tr)
        if tr_idx == 0:
            raise ValueError('no tr above topmost tr')
        return tr_lst[tr_idx-1]