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

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

    addNode(N2);
    Set<String> inEdges = network.inEdges(N2);
    assertThrows(UnsupportedOperationException.class, () -> inEdges.add(E12));

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

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

    addNode(N2);
    Set<String> inEdges = network.inEdges(N2);
    assertThrows(UnsupportedOperationException.class, () -> inEdges.add(E12));

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

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

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

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