public void successors_oneEdge() {
    putEdge(N1, N2);
    assertThat(graph.successors(N1)).containsExactly(N2);
    assertThat(graph.successors(N2)).containsExactly(N1);
  }

  @Test
  public void incidentEdges_oneEdge() {
    putEdge(N1, N2);
    EndpointPair<Integer> expectedEndpoints = EndpointPair.unordered(N1, N2);
    assertThat(graph.incidentEdges(N1)).containsExactly(expectedEndpoints);

  public void successors_oneEdge() {
    putEdge(N1, N2);
    assertThat(graph.successors(N1)).containsExactly(N2);
    assertThat(graph.successors(N2)).containsExactly(N1);
  }

  @Test
  public void incidentEdges_oneEdge() {
    putEdge(N1, N2);
    EndpointPair<Integer> expectedEndpoints = EndpointPair.unordered(N1, N2);
    assertThat(graph.incidentEdges(N1)).containsExactly(expectedEndpoints);

    // Network with no edges, given disconnected nodes
    addNode(N1);
    addNode(N2);
    assertThat(network.edges()).isEmpty();
  }

  @Test
  public void incidentEdges_oneEdge() {
    addEdge(N1, N2, E12);
    assertThat(network.incidentEdges(N2)).containsExactly(E12);
    assertThat(network.incidentEdges(N1)).containsExactly(E12);
  }

  @Test