assertThat(network.nodes()).containsExactly("A", "B");
    assertThat(network.edges()).containsExactly(10);
    assertThat(network.incidentNodes(10)).isEqualTo(EndpointPair.ordered("A", "B"));
  }

  @Test
  public void immutableNetworkBuilder_putEdgeFromEndpointPair() {
    ImmutableNetwork<String, Integer> network =
        NetworkBuilder.directed()

            });
    assertThat(e).hasMessageThat().contains(ENDPOINTS_MISMATCH);
  }

  @Override
  @Test
  public void incidentNodes_oneEdge() {
    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(e).hasMessageThat().contains(ENDPOINTS_MISMATCH);
  }

  @Override
  @Test
  public void incidentNodes_oneEdge() {
    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);

          @Override
          public Iterator<EndpointPair<N>> iterator() {
            return Iterators.transform(
                AbstractNetwork.this.edges().iterator(), edge -> incidentNodes(edge));
          }

          @Override
          public int size() {
            return AbstractNetwork.this.edges().size();
          }

    return (E edge) -> network.incidentNodes(edge).source();
  }

  private static <N, E> Function<E, N> targetNodeFn(Network<N, E> network) {
    return (E edge) -> network.incidentNodes(edge).target();
  }

  private static <N, E> Function<E, N> adjacentNodeFn(Network<N, E> network, N node) {
    return (E edge) -> network.incidentNodes(edge).adjacentNode(node);
  }

  /**

    return delegate().inEdges(node);
  }

  @Override
  public Set<E> outEdges(N node) {
    return delegate().outEdges(node);
  }

  @Override
  public EndpointPair<N> incidentNodes(E edge) {
    return delegate().incidentNodes(edge);
  }

  @Override
  public Set<E> adjacentEdges(E edge) {
    return delegate().adjacentEdges(edge);
  }

  @Override
  public int degree(N node) {

  }

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

  @Test
  public void incidentNodes_edgeNotInGraph() {
    assertEdgeNotInGraphErrorMessage(
        assertThrows(
            IllegalArgumentException.class, () -> network.incidentNodes(EDGE_NOT_IN_GRAPH)));
  }

  @Test

  }

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

  @Test
  public void incidentNodes_edgeNotInGraph() {
    assertEdgeNotInGraphErrorMessage(
        assertThrows(
            IllegalArgumentException.class, () -> network.incidentNodes(EDGE_NOT_IN_GRAPH)));
  }

  @Test

    }

    @Override
    public Set<E> outEdges(N node) {
      return delegate().inEdges(node); // transpose
    }

    @Override
    public EndpointPair<N> incidentNodes(E edge) {
      EndpointPair<N> endpointPair = delegate().incidentNodes(edge);
      return EndpointPair.of(network, endpointPair.nodeV(), endpointPair.nodeU()); // transpose
    }

    @Override
    public Set<E> edgesConnecting(N nodeU, N nodeV) {

  private final ElementOrder<E> edgeOrder;

  final MapIteratorCache<N, NetworkConnections<N, E>> nodeConnections;

  // We could make this a Map<E, EndpointPair<N>>. It would make incidentNodes(edge) slightly
  // faster, but also make Networks consume 5 to 20+% (increasing with average degree) more memory.
  final MapIteratorCache<E, N> edgeToReferenceNode; // referenceNode == source if directed