}

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

    addEdge(N1, N2, E12);
    Set<String> adjacentEdges = network.adjacentEdges(E12);
    try {
      adjacentEdges.add(E23);
      fail(ERROR_MODIFIABLE_COLLECTION);
    } catch (UnsupportedOperationException e) {

  }

  @Override
  public void adjacentEdges_checkReturnedSetMutability() {
    addEdge(N1, N2, E12);
    Set<String> adjacentEdges = network.adjacentEdges(E12);
    try {
      adjacentEdges.add(E23);
      fail(ERROR_MODIFIABLE_COLLECTION);
    } catch (UnsupportedOperationException e) {
      addEdge(N2, N3, E23);
      assertThat(network.adjacentEdges(E12)).containsExactlyElementsIn(adjacentEdges);
    }

  }

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

    addEdge(N1, N2, E12);
    Set<String> adjacentEdges = network.adjacentEdges(E12);
    try {
      adjacentEdges.add(E23);
      fail(ERROR_MODIFIABLE_COLLECTION);
    } catch (UnsupportedOperationException e) {

  }

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

  @Test
  public void adjacentEdges_parallelEdges() {
    assume().that(network.allowsParallelEdges()).isTrue();

    addEdge(N1, N2, E12);
    addEdge(N1, N2, E12_A);

  }

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

  @Test
  public void adjacentEdges_parallelEdges() {
    assume().that(network.allowsParallelEdges()).isTrue();

    addEdge(N1, N2, E12);
    addEdge(N1, N2, E12_A);

  }

  @Override
  public void adjacentEdges_checkReturnedSetMutability() {
    addEdge(N1, N2, E12);
    Set<String> adjacentEdges = network.adjacentEdges(E12);
    try {
      adjacentEdges.add(E23);
      fail(ERROR_MODIFIABLE_COLLECTION);
    } catch (UnsupportedOperationException e) {
      addEdge(N2, N3, E23);
      assertThat(network.adjacentEdges(E12)).containsExactlyElementsIn(adjacentEdges);
    }

    }

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

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

    // _don't_ override edge*Connecting*; we want the behavior from AbstractNetwork
  }

    }

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

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

    // _don't_ override edge*Connecting*; we want the behavior from AbstractNetwork
  }

  }

  @Override
  public int outDegree(N node) {
    return isDirected() ? outEdges(node).size() : degree(node);
  }

  @Override
  public Set<E> adjacentEdges(E edge) {
    EndpointPair<N> endpointPair = incidentNodes(edge); // Verifies that edge is in this network.
    Set<E> endpointPairIncidentEdges =

   *       behavior is undefined
   * </ul>
   *
   * @throws IllegalArgumentException if {@code edge} is not an element of this network
   */
  Set<E> adjacentEdges(E edge);

  /**
   * Returns a live view of the set of edges that each directly connect {@code nodeU} to {@code
   * nodeV}.
   *