private static final EndpointPair<Integer> ENDPOINTS_N1N2 = EndpointPair.ordered(N1, N2);
  private static final EndpointPair<Integer> ENDPOINTS_N2N1 = EndpointPair.ordered(N2, N1);

  @After
  public void validateUndirectedEdges() {
    for (Integer node : network.nodes()) {
      new EqualsTester()
          .addEqualityGroup(
              network.inEdges(node), network.outEdges(node), network.incidentEdges(node))
          .testEquals();