network.addEdge(N1, N2, E12);
    assertThat(networkForTest.edgesConnecting(N1, N2)).containsExactlyElementsIn(edgesConnecting);
  }

  @Test
  public void edgesConnecting_oneEdge() {
    network.addEdge(N1, N2, E12);
    assertThat(networkForTest.edgesConnecting(N1, N2)).containsExactly(E12);
    if (edgeType == EdgeType.DIRECTED) {

    addEdge(N1, N2, E12);
    assertThat(network.incidentNodes(E12).source()).isEqualTo(N1);
    assertThat(network.incidentNodes(E12).target()).isEqualTo(N2);
  }

  @Test
  public void edgesConnecting_oneEdge() {
    addEdge(N1, N2, E12);
    assertThat(network.edgesConnecting(N1, N2)).containsExactly(E12);
    // Passed nodes should be in the correct edge direction, first is the

    addEdge(N1, N2, E12);
    assertThat(network.incidentNodes(E12).source()).isEqualTo(N1);
    assertThat(network.incidentNodes(E12).target()).isEqualTo(N2);
  }

  @Test
  public void edgesConnecting_oneEdge() {
    addEdge(N1, N2, E12);
    assertThat(network.edgesConnecting(N1, N2)).containsExactly(E12);
    // Passed nodes should be in the correct edge direction, first is the

              String unused = network.edgeConnectingOrNull(ENDPOINTS_N1N2);
            });
    assertThat(e).hasMessageThat().contains(ENDPOINTS_MISMATCH);
  }

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

  @Test