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.

def merge(self, other_tc):
        """
        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.
        """
        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