Graph graph3 = new Graph();
        addEdge(graph3, "a", "b");
        addEdge(graph3, "b", "c");
        addEdge(graph3, "b", "d");
        addEdge(graph3, "a", "d");

        Graph graph4 = new Graph();
        addEdge(graph4, "a", "b");
        addEdge(graph4, "b", "c");
        addEdge(graph4, "b", "d");
        addEdge(graph4, "a", "d");

        Graph.Vertex ep3 = graph.addVertex(AFTER + phase.name());
        graph.addEdge(ep0, ep1);
        graph.addEdge(ep1, ep2);
        graph.addEdge(ep2, ep3);
        if (before != null) {
            graph.addEdge(before, ep0);
        }
        if (after != null) {
            graph.addEdge(ep3, after);
        }
        phase.links().forEach(link -> {

    addEdge(N1, N1, E11);
    addEdge(N1, N2, E12);
    assertThat(network.adjacentNodes(N1)).containsExactly(N1, N2);
  }

  @Test
  public void adjacentEdges_selfLoop() {
    assume().that(network.allowsSelfLoops()).isTrue();

    addEdge(N1, N1, E11);
    addEdge(N1, N2, E12);
    assertThat(network.adjacentEdges(E11)).containsExactly(E12);
  }

  @Test

    directedGraph.addEdge(N1, N2, E12);
    directedGraph.addEdge(N1, N2, E12_A);
    directedGraph.addEdge(N1, N1, E11);
    directedGraph.addEdge(N3, N4, E34);

    MutableNetwork<Integer, String> expectedTranspose =
        NetworkBuilder.directed().allowsParallelEdges(true).allowsSelfLoops(true).build();
    expectedTranspose.addEdge(N3, N1, E13);
    expectedTranspose.addEdge(N1, N3, E31);
    expectedTranspose.addEdge(N2, N1, E12);

  }

  @Test
  public void edgesConnecting_directed() {
    MutableNetwork<String, String> mutableNetwork =
        NetworkBuilder.directed().allowsSelfLoops(true).build();
    mutableNetwork.addEdge("A", "A", "AA");
    mutableNetwork.addEdge("A", "B", "AB");
    Network<String, String> network = ImmutableNetwork.copyOf(mutableNetwork);

    assertThat(network.edgesConnecting("A", "A")).containsExactly("AA");

    MutableNetwork<String, Integer> network = NetworkBuilder.directed().build();
    network.addEdge("a", "b", 11);

    Traverser.forTree(network); // Does not throw
  }

  @Test
  public void forTree_withUndirectedNetwork_throws() throws Exception {
    MutableNetwork<String, Integer> network = NetworkBuilder.undirected().build();
    network.addEdge("a", "b", 11);