@Override
  @Test
  public void inEdges_checkReturnedSetMutability() {
    addNode(N2);
    Set<String> inEdges = network.inEdges(N2);
    assertThrows(UnsupportedOperationException.class, () -> inEdges.add(E12));
    addEdge(N1, N2, E12);
    assertThat(network.inEdges(N2)).containsExactlyElementsIn(inEdges);
  }

  @Override
  @Test
  public void outEdges_checkReturnedSetMutability() {
    addNode(N1);

            .contains(inEdge);
        if (network.isDirected()) {
          assertThat(network.incidentNodes(inEdge).target()).isEqualTo(node);
        }
      }

      for (E outEdge : sanityCheckSet(network.outEdges(node))) {
        assertThat(network.incidentEdges(node)).contains(outEdge);

    assume().that(graphIsMutable()).isTrue();

    addNode(N2);
    Set<String> inEdges = network.inEdges(N2);
    assertThrows(UnsupportedOperationException.class, () -> inEdges.add(E12));
    addEdge(N1, N2, E12);
    assertThat(network.inEdges(N2)).containsExactlyElementsIn(inEdges);
  }

  @Override
  @Test
  public void outEdges_checkReturnedSetMutability() {

    assume().that(graphIsMutable()).isTrue();

    addNode(N2);
    Set<String> inEdges = network.inEdges(N2);
    assertThrows(UnsupportedOperationException.class, () -> inEdges.add(E12));
    addEdge(N1, N2, E12);
    assertThat(network.inEdges(N2)).containsExactlyElementsIn(inEdges);
  }

  @Override
  @Test
  public void outEdges_checkReturnedSetMutability() {

 */
final class DirectedMultiNetworkConnections<N, E> extends AbstractDirectedNetworkConnections<N, E> {

  private DirectedMultiNetworkConnections(
      Map<E, N> inEdges, Map<E, N> outEdges, int selfLoopCount) {
    super(inEdges, outEdges, selfLoopCount);
  }

  static <N, E> DirectedMultiNetworkConnections<N, E> of() {
    return new DirectedMultiNetworkConnections<>(

 */
final class DirectedMultiNetworkConnections<N, E> extends AbstractDirectedNetworkConnections<N, E> {

  private DirectedMultiNetworkConnections(
      Map<E, N> inEdges, Map<E, N> outEdges, int selfLoopCount) {
    super(inEdges, outEdges, selfLoopCount);
  }

  static <N, E> DirectedMultiNetworkConnections<N, E> of() {
    return new DirectedMultiNetworkConnections<>(

  }

  static <N, E> DirectedNetworkConnections<N, E> ofImmutable(
      Map<E, N> inEdges, Map<E, N> outEdges, int selfLoopCount) {
    return new DirectedNetworkConnections<>(
        ImmutableBiMap.copyOf(inEdges), ImmutableBiMap.copyOf(outEdges), selfLoopCount);
  }

  @Override
  public Set<N> predecessors() {

  }

  static <N, E> DirectedNetworkConnections<N, E> ofImmutable(
      Map<E, N> inEdges, Map<E, N> outEdges, int selfLoopCount) {
    return new DirectedNetworkConnections<>(
        ImmutableBiMap.copyOf(inEdges), ImmutableBiMap.copyOf(outEdges), selfLoopCount);
  }

  @Override
  public Set<N> predecessors() {

    return delegate().successors(node);
  }

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

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

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

  @Override
  public EndpointPair<N> incidentNodes(E edge) {

    return adjacentNodes();
  }

  @Override
  public Set<E> incidentEdges() {
    return Collections.unmodifiableSet(incidentEdgeMap.keySet());
  }

  @Override
  public Set<E> inEdges() {
    return incidentEdges();
  }

  @Override
  public Set<E> outEdges() {
    return incidentEdges();
  }

  @Override
  public N adjacentNode(E edge) {