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

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

  @Test
  public void nodes_noNodes() {

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

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

  @Test
  public void nodes_noNodes() {

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

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