'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):

    (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
