assertThrows(UnsupportedOperationException.class, () -> nodes.add(N2));
    addNode(N1);
    assertThat(graph.nodes()).containsExactlyElementsIn(nodes);
  }

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

    addNode(N1);
    Set<Integer> adjacentNodes = graph.adjacentNodes(N1);

    assertThrows(UnsupportedOperationException.class, () -> nodes.add(N2));
    addNode(N1);
    assertThat(graph.nodes()).containsExactlyElementsIn(nodes);
  }

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

    addNode(N1);
    Set<Integer> adjacentNodes = graph.adjacentNodes(N1);

    assertThrows(UnsupportedOperationException.class, () -> nodes.add(N2));
    addNode(N1);
    assertThat(graph.nodes()).containsExactlyElementsIn(nodes);
  }

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

    addNode(N1);
    Set<Integer> adjacentNodes = graph.adjacentNodes(N1);

    addEdge(N1, N2, E12);
    assertThat(network.incidentEdges(N1)).containsExactlyElementsIn(incidentEdges);
  }

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

    addNode(N1);
    Set<Integer> adjacentNodes = network.adjacentNodes(N1);

    addEdge(N1, N2, E12);
    assertThat(network.incidentEdges(N1)).containsExactlyElementsIn(incidentEdges);
  }

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

    addNode(N1);
    Set<Integer> adjacentNodes = network.adjacentNodes(N1);

    addEdge(N1, N2, E12);
    assertThat(network.incidentEdges(N1)).containsExactlyElementsIn(incidentEdges);
  }

  @Override
  @Test
  public void adjacentNodes_checkReturnedSetMutability() {
    addNode(N1);
    Set<Integer> adjacentNodes = network.adjacentNodes(N1);
    assertThrows(UnsupportedOperationException.class, () -> adjacentNodes.add(N2));
    addEdge(N1, N2, E12);

   * Verifies that the {@code Set} returned by {@code adjacentNodes} has the expected mutability
   * property (see the {@code Network} documentation for more information).
   */
  @Test
  public abstract void adjacentNodes_checkReturnedSetMutability();

  /**
   * Verifies that the {@code Set} returned by {@code adjacentEdges} has the expected mutability
   * property (see the {@code Network} documentation for more information).
   */
  @Test