code: def merge(self, other_tc):
          
          top, left, height, width = self._span_dimensions(other_tc)
          top_tc = self._tbl.tr_lst[top].tc_at_grid_col(left)
          top_tc._grow_to(width, height)
          return top_tc
docs: Return the top-left ``<w:tc>`` element of a new span formed by
          merging the rectangular region defined by using this tc element and
          *other_tc* as diagonal corners.