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 canonLinkKey() {
        LinkKey fb = TopoUtils.canonicalLinkKey(LINK_FU_BAH);
        LinkKey bf = TopoUtils.canonicalLinkKey(LINK_BAH_FU);
        assertEquals("not canonical", fb, bf);
    }