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

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

    addNode(N1);
    Set<String> outEdges = network.outEdges(N1);
    assertThrows(UnsupportedOperationException.class, () -> outEdges.add(E12));

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

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

    addNode(N1);
    Set<String> outEdges = network.outEdges(N1);
    assertThrows(UnsupportedOperationException.class, () -> outEdges.add(E12));

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

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

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

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

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

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