}

  @Override
  @Test
  public void edgesConnecting_checkReturnedSetMutability() {
    addNode(N1);
    addNode(N2);
    Set<String> edgesConnecting = network.edgesConnecting(N1, N2);
    UnsupportedOperationException e =
        assertThrows(UnsupportedOperationException.class, () -> edgesConnecting.add(E23));
    addEdge(N1, N2, E12);
    assertThat(network.edgesConnecting(N1, N2)).containsExactlyElementsIn(edgesConnecting);

          + "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 "

    }
  }

  @Override
  @Test
  public void edgesConnecting_checkReturnedSetMutability() {
    assume().that(graphIsMutable()).isTrue();

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

    assertThat(transpose(transpose)).isSameInstanceAs(directedGraph);
    AbstractNetworkTest.validateNetwork(transpose);

    assertThat(transpose.edgesConnecting(N1, N2)).isEmpty();
    assertThat(transpose.edgeConnecting(N1, N2).isPresent()).isFalse();
    assertThat(transpose.edgeConnectingOrNull(N1, N2)).isNull();

    for (Integer node : directedGraph.nodes()) {

      }

      for (N otherNode : network.nodes()) {
        Set<E> edgesConnecting = sanityCheckSet(network.edgesConnecting(node, otherNode));
        switch (edgesConnecting.size()) {
          case 0:
            assertThat(network.edgeConnectingOrNull(node, otherNode)).isNull();
            assertThat(network.edgeConnecting(node, otherNode).isPresent()).isFalse();

   *
   * <p>In an undirected network, this is equal to {@code edgesConnecting(nodeV, nodeU)}.
   *
   * <p>The resulting set of edges will be parallel (i.e. have equal {@link
   * #incidentNodes(Object)}). If this network does not {@link #allowsParallelEdges() allow parallel
   * edges}, the resulting set will contain at most one edge (equivalent to {@code
   * edgeConnecting(nodeU, nodeV).asSet()}).
   *

 *
 * @author James Sexton
 * @param <E> Edge parameter type
 */
@ElementTypesAreNonnullByDefault
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);
  }

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

  @Test
  public void edgesConnecting_undirected() {
    MutableNetwork<String, String> mutableNetwork =
        NetworkBuilder.undirected().allowsSelfLoops(true).build();
    mutableNetwork.addEdge("A", "A", "AA");
    mutableNetwork.addEdge("A", "B", "AB");
    Network<String, String> network = ImmutableNetwork.copyOf(mutableNetwork);

    assertNodeNotInGraphErrorMessage(e);
  }

  @Test
  public void edgesConnecting_checkReturnedSetMutability() {
    network.addNode(N1);
    network.addNode(N2);
    Set<String> edgesConnecting = network.edgesConnecting(N1, N2);
    UnsupportedOperationException e =
        assertThrows(UnsupportedOperationException.class, () -> edgesConnecting.add(E23));

  }

  @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);
  }