def key_desc(name):
        'if name is a string key, use the larger size of r1 or r2 before merging'
        dt1 = r1.dtype[name]
        if dt1.type != np.string_:
            return (name, dt1.descr[0][1])

        dt2 = r1.dtype[name]
        assert dt2==dt1
        if dt1.num>dt2.num:
            return (name, dt1.descr[0][1])
        else:
            return (name, dt2.descr[0][1])