public static LinkKey canonicalLinkKey(Link link) {
        String sn = link.src().elementId().toString();
        String dn = link.dst().elementId().toString();
        return sn.compareTo(dn) < 0 ?
                linkKey(link.src(), link.dst()) : linkKey(link.dst(), link.src());
    }
@Test
    public void canon723() {
        LinkKey lk1 = TopoUtils.canonicalLinkKey(LINK_7_TO_3);
        print(lk1);
        LinkKey lk2 = TopoUtils.canonicalLinkKey(LINK_3_TO_7);
        print(lk2);
        assertEquals("not canonical 3/7", lk1, lk2);
    }