docs: 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]