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) {

  @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);

      }

      @Override
      public boolean contains(@Nullable Object obj) {
        return inEdgeMap.containsKey(obj) || outEdgeMap.containsKey(obj);
      }
    };
  }

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

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

  @Override

  public Set<E> edgesConnecting(N nodeU, N nodeV) {
    Set<E> outEdgesU = outEdges(nodeU);
    Set<E> inEdgesV = inEdges(nodeV);
    return nodePairInvalidatableSet(
        outEdgesU.size() <= inEdgesV.size()
            ? unmodifiableSet(Sets.filter(outEdgesU, connectedPredicate(nodeU, nodeV)))
            : unmodifiableSet(Sets.filter(inEdgesV, connectedPredicate(nodeV, nodeU))),
        nodeU,
        nodeV);
  }

  @Override

  public Set<E> edgesConnecting(N nodeU, N nodeV) {
    Set<E> outEdgesU = outEdges(nodeU);
    Set<E> inEdgesV = inEdges(nodeV);
    return nodePairInvalidatableSet(
        outEdgesU.size() <= inEdgesV.size()
            ? unmodifiableSet(Sets.filter(outEdgesU, connectedPredicate(nodeU, nodeV)))
            : unmodifiableSet(Sets.filter(inEdgesV, connectedPredicate(nodeV, nodeU))),
        nodeU,
        nodeV);
  }

  @Override

    return nodePairInvalidatableSet(connectionsU.edgesConnecting(nodeV), nodeU, nodeV);
  }

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

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

  @Override

    return nodePairInvalidatableSet(connectionsU.edgesConnecting(nodeV), nodeU, nodeV);
  }

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

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

  @Override

 * @param <E> Edge parameter type
 */
interface NetworkConnections<N, E> {

  Set<N> adjacentNodes();

  Set<N> predecessors();

  Set<N> successors();

  Set<E> incidentEdges();

  Set<E> inEdges();

  Set<E> outEdges();

  /**
   * Returns the set of edges connecting the origin node to {@code node}. For networks without
   * parallel edges, this set cannot be of size greater than one.
   */

  //

  /**
   * Returns a live view of the edges whose {@link #incidentNodes(Object) incident nodes} in this
   * network include {@code node}.
   *
   * <p>This is equal to the union of {@link #inEdges(Object)} and {@link #outEdges(Object)}.
   *
   * <p>If {@code node} is removed from the network after this method is called, the {@code Set}
   * {@code view} returned by this method will be invalidated, and will throw {@code