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

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

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

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

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

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