*
 * @author James Sexton
 * @param <E> Edge parameter type
 */
final class EdgesConnecting<E> extends AbstractSet<E> {

  private final Map<?, E> nodeToOutEdge;
  private final Object targetNode;

  EdgesConnecting(Map<?, E> nodeToEdgeMap, Object targetNode) {
    this.nodeToOutEdge = checkNotNull(nodeToEdgeMap);
    this.targetNode = checkNotNull(targetNode);
  }

 *
 * @author James Sexton
 * @param <E> Edge parameter type
 */
final class EdgesConnecting<E> extends AbstractSet<E> {

  private final Map<?, E> nodeToOutEdge;
  private final Object targetNode;

  EdgesConnecting(Map<?, E> nodeToEdgeMap, Object targetNode) {
    this.nodeToOutEdge = checkNotNull(nodeToEdgeMap);
    this.targetNode = checkNotNull(targetNode);
  }

  }

  @Override
  public Set<E> edgesConnecting(N nodeU, N nodeV) {
    return delegate().edgesConnecting(nodeU, nodeV);
  }

  @Override
  public Set<E> edgesConnecting(EndpointPair<N> endpoints) {
    return delegate().edgesConnecting(endpoints);
  }

  @Override
  public Optional<E> edgeConnecting(N nodeU, N nodeV) {
    return delegate().edgeConnecting(nodeU, nodeV);
  }

  @Override

  }

  @Override
  public Set<N> adjacentNodes() {
    return Collections.unmodifiableSet(((BiMap<E, N>) incidentEdgeMap).values());
  }

  @Override
  public Set<E> edgesConnecting(N node) {
    return new EdgesConnecting<>(((BiMap<E, N>) incidentEdgeMap).inverse(), node);
  }

    addNode(N1);
    addNode(N2);
    Set<String> edgesConnecting = network.edgesConnecting(N1, N2);
    assertThrows(UnsupportedOperationException.class, () -> edgesConnecting.add(E23));
    addEdge(N1, N2, E12);
    assertThat(network.edgesConnecting(N1, N2)).containsExactlyElementsIn(edgesConnecting);
  }

  @Override
  @Test
  public void inEdges_checkReturnedSetMutability() {

    Set<E> edgesConnecting = edgesConnecting(nodeU, nodeV);
    switch (edgesConnecting.size()) {
      case 0:
        return null;
      case 1:
        return edgesConnecting.iterator().next();
      default:
        throw new IllegalArgumentException(String.format(MULTIPLE_EDGES_CONNECTING, nodeU, nodeV));
    }
  }

  @Override
  public @Nullable E edgeConnectingOrNull(EndpointPair<N> endpoints) {

    assertThat(directedGraph.edges()).isEmpty();
    assertThat(directedGraph.addEdge(N1, N2, E12)).isTrue();
    assertThat(directedGraph.edgesConnecting(N1, N2)).isEqualTo(ImmutableSet.of(E12));
    assertThat(directedGraph.edgesConnecting(N2, N1)).isEmpty();

    // By default, parallel edges are not allowed.
    IllegalArgumentException e =

    mutableNetwork.addEdge("A", "B", "AB");
    Network<String, String> network = ImmutableNetwork.copyOf(mutableNetwork);

    assertThat(network.edgesConnecting("A", "A")).containsExactly("AA");
    assertThat(network.edgesConnecting("A", "B")).containsExactly("AB");
    assertThat(network.edgesConnecting("B", "A")).isEmpty();
  }

  @Test
  public void edgesConnecting_undirected() {
    MutableNetwork<String, String> mutableNetwork =

  }

  @Override
  public @Nullable E edgeConnectingOrNull(N nodeU, N nodeV) {
    Set<E> edgesConnecting = edgesConnecting(nodeU, nodeV);
    switch (edgesConnecting.size()) {
      case 0:
        return null;
      case 1:
        return edgesConnecting.iterator().next();
      default:
        throw new IllegalArgumentException(String.format(MULTIPLE_EDGES_CONNECTING, nodeU, nodeV));
    }

          + "so it cannot be reused to connect the following nodes: %s.";
  static final String MULTIPLE_EDGES_CONNECTING =
      "Cannot call edgeConnecting() when parallel edges exist between %s and %s. Consider calling "
          + "edgesConnecting() instead.";
  static final String PARALLEL_EDGES_NOT_ALLOWED =
      "Nodes %s and %s are already connected by a different edge. To construct a graph "