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

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

  @Test
  public void nodes_noNodes() {