putEdge(N1, N2);
    assertThat(graph.predecessors(N2)).containsExactlyElementsIn(predecessors);
  }

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

    addNode(N1);
    Set<Integer> successors = graph.successors(N1);
    assertThrows(UnsupportedOperationException.class, () -> successors.add(N2));

    putEdge(N1, N2);
    assertThat(graph.predecessors(N2)).containsExactlyElementsIn(predecessors);
  }

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

    addNode(N1);
    Set<Integer> successors = graph.successors(N1);
    assertThrows(UnsupportedOperationException.class, () -> successors.add(N2));

    putEdge(N1, N2);
    assertThat(graph.predecessors(N2)).containsExactlyElementsIn(predecessors);
  }

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

    addNode(N1);
    Set<Integer> successors = graph.successors(N1);
    assertThrows(UnsupportedOperationException.class, () -> successors.add(N2));

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

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

    addNode(N1);
    Set<Integer> successors = network.successors(N1);
    assertThrows(UnsupportedOperationException.class, () -> successors.add(N2));

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

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

    addNode(N1);
    Set<Integer> successors = network.successors(N1);
    assertThrows(UnsupportedOperationException.class, () -> successors.add(N2));

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

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

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

  @Test
  public void nodes_oneNode() {
    addNode(N1);
    assertThat(network.nodes()).containsExactly(N1);
  }

  @Test
  public void nodes_noNodes() {