'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.'

def merge(self, other_cell):

    (tc, tc_2) = (self._tc, other_cell._tc)
    merged_tc = tc.merge(tc_2)
    return _Cell(merged_tc, self._parent)
