expectedClosure.putEdge(N1, N3);
    expectedClosure.putEdge(N1, N4);
    expectedClosure.putEdge(N2, N1);
    expectedClosure.putEdge(N2, N2);
    expectedClosure.putEdge(N2, N3);
    expectedClosure.putEdge(N2, N4);
    expectedClosure.putEdge(N3, N1);
    expectedClosure.putEdge(N3, N2);
    expectedClosure.putEdge(N3, N3);
    expectedClosure.putEdge(N3, N4);
    expectedClosure.putEdge(N4, N1);

    directedGraph.putEdge(N1, N2);
    directedGraph.putEdge(N2, N1);
    directedGraph.putEdge(N1, N3);
    directedGraph.putEdge(N4, N4);
    containsExactlySanityCheck(
        directedGraph.edges(),
        EndpointPair.ordered(N1, N2),
        EndpointPair.ordered(N2, N1),
        EndpointPair.ordered(N1, N3),
        EndpointPair.ordered(N4, N4));
  }

  @Test
  public void endpointPair_undirectedGraph() {

   *
   * <pre>
   * Nodes = {N1, N2, N3, N4}
   *    N2           __
   *   /  \         |  |
   * N1----N3      N4__|
   *
   * Visited Nodes = {}
   * EndpointPair [N1, N2] - return
   * EndpointPair [N1, N3] - return
   * Visited Nodes = {N1}
   * EndpointPair [N2, N1] - skip
   * EndpointPair [N2, N3] - return
   * Visited Nodes = {N1, N2}
   * EndpointPair [N3, N1] - skip
   * EndpointPair [N3, N2] - skip

    assertTrue(graphAsMutableGraph.putEdge(N4, N1));
    assertTrue(graphAsMutableGraph.putEdge(N2, N3));
    assertThat(graph.nodes()).containsExactly(N1, N5, N4, N2, N3).inOrder();
    assertThat(graph.adjacentNodes(N1)).containsExactly(N4, N5);
    assertThat(graph.adjacentNodes(N2)).containsExactly(N3);
    assertThat(graph.adjacentNodes(N3)).containsExactly(N2);
    assertThat(graph.adjacentNodes(N4)).containsExactly(N1);

    assertTrue(graphAsMutableGraph.putEdge(N4, N1));
    assertTrue(graphAsMutableGraph.putEdge(N2, N3));
    assertThat(graph.nodes()).containsExactly(N1, N5, N4, N2, N3).inOrder();
    assertThat(graph.successors(N1)).containsExactly(N5);
    assertThat(graph.successors(N2)).containsExactly(N3);
    assertThat(graph.successors(N3)).isEmpty();
    assertThat(graph.successors(N4)).containsExactly(N1);

    assertTrue(networkAsMutableNetwork.addEdge(N2, N3, E23));
    assertThat(network.nodes()).containsExactly(N1, N5, N4, N2, N3);
    assertThat(network.edges()).containsExactly(E15, E41, E23);
    assertThat(network.edgesConnecting(N1, N5)).containsExactly(E15);
    assertThat(network.edgesConnecting(N4, N1)).containsExactly(E41);
    assertThat(network.edgesConnecting(N2, N3)).containsExactly(E23);

    assertTrue(networkAsMutableNetwork.addEdge(N2, N3, E23));
    assertThat(network.nodes()).containsExactly(N1, N5, N4, N2, N3);
    assertThat(network.edges()).containsExactly(E15, E41, E23);
    assertThat(network.edgesConnecting(N1, N5)).containsExactly(E15);
    assertThat(network.edgesConnecting(N4, N1)).containsExactly(E41);
    assertThat(network.edgesConnecting(N2, N3)).containsExactly(E23);

  @Test
  public void adjacentEdges_bothEndpoints() {
    addEdge(N1, N2, E12);
    addEdge(N2, N3, E23);
    addEdge(N3, N1, E31);
    addEdge(N3, N4, E34);
    assertThat(network.adjacentEdges(E12)).containsExactly(E31, E23);
  }

  @Test
  public void adjacentEdges_noAdjacentEdges() {
    addEdge(N1, N2, E12);
    addEdge(N3, N4, E34);
    assertThat(network.adjacentEdges(E12)).isEmpty();
  }

  @Test

    MutableGraph<Integer> g1 = builder.build();
    MutableGraph<Integer> g2 = builder.build();

    // for g1, add 1->2 first, then 3->1
    g1.putEdge(N1, N2);
    g1.putEdge(N3, N1);

    // for g2, add 3->1 first, then 1->2
    g2.putEdge(N3, N1);
    g2.putEdge(N1, N2);

    assertThat(g1).isEqualTo(g2);
  }

  @Test
  public void equivalent_edgeDirectionsDiffer() {
    graph.putEdge(N1, N2);

    MutableGraph<Integer> g1 = builder.build();
    MutableGraph<Integer> g2 = builder.build();

    // for g1, add 1->2 first, then 3->1
    g1.putEdge(N1, N2);
    g1.putEdge(N3, N1);

    // for g2, add 3->1 first, then 1->2
    g2.putEdge(N3, N1);
    g2.putEdge(N1, N2);

    assertThat(g1).isEqualTo(g2);
  }

  @Test
  public void equivalent_edgeDirectionsDiffer() {
    graph.putEdge(N1, N2);