docs: def merge(self, other_cell):
          """
          Return a merged cell created by spanning the rectangular region
          having this cell and *other_cell* as diagonal corners. Raises
          |InvalidSpanError| if the cells do not define a rectangular region.
          """
          tc, tc_2 = self._tc, other_cell._tc
          merged_tc = tc.merge(tc_2)
          return _Cell(merged_tc, self._parent)