}

    directedGraph.addEdge(N2, N1, E21);
    // View should be updated.
    assertThat(transpose.edgesConnecting(N1, N2)).containsExactly(E21);
    assertThat(transpose.edgeConnecting(N1, N2)).hasValue(E21);
    assertThat(transpose.edgeConnectingOrNull(N1, N2)).isEqualTo(E21);
    AbstractNetworkTest.validateNetwork(transpose);
  }

  @Test
  public void inducedSubgraph_graph() {