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

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

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

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

    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);