addNode(N1);
    addNode(N2);
    Set<String> edgesConnecting = network.edgesConnecting(N1, N2);
    assertThrows(UnsupportedOperationException.class, () -> edgesConnecting.add(E23));
    addEdge(N1, N2, E12);
    assertThat(network.edgesConnecting(N1, N2)).containsExactlyElementsIn(edgesConnecting);
  }

  @Override
  @Test
  public void inEdges_checkReturnedSetMutability() {

    addNode(N1);
    addNode(N2);
    Set<String> edgesConnecting = network.edgesConnecting(N1, N2);
    assertThrows(UnsupportedOperationException.class, () -> edgesConnecting.add(E23));
    addEdge(N1, N2, E12);
    assertThat(network.edgesConnecting(N1, N2)).containsExactlyElementsIn(edgesConnecting);
  }

  @Override
  @Test
  public void inEdges_checkReturnedSetMutability() {

    addNode(N1);
    addNode(N2);
    Set<String> edgesConnecting = network.edgesConnecting(N1, N2);
    assertThrows(UnsupportedOperationException.class, () -> edgesConnecting.add(E23));
    addEdge(N1, N2, E12);
    assertThat(network.edgesConnecting(N1, N2)).containsExactlyElementsIn(edgesConnecting);
  }

  @Override
  @Test
  public void inEdges_checkReturnedSetMutability() {

    addNode(N1);
    addNode(N2);
    Set<String> edgesConnecting = network.edgesConnecting(N1, N2);
    assertThrows(UnsupportedOperationException.class, () -> edgesConnecting.add(E23));
    addEdge(N1, N2, E12);
    assertThat(network.edgesConnecting(N1, N2)).containsExactlyElementsIn(edgesConnecting);
  }

  @Override
  @Test
  public void inEdges_checkReturnedSetMutability() {

    assertThat(directedGraph.edges()).isEmpty();
    assertThat(directedGraph.addEdge(N1, N2, E12)).isTrue();
    assertThat(directedGraph.edgesConnecting(N1, N2)).isEqualTo(ImmutableSet.of(E12));
    assertThat(directedGraph.edgesConnecting(N2, N1)).isEmpty();

    // By default, parallel edges are not allowed.
    IllegalArgumentException e =

   *
   * <p>In an undirected network, this is equal to {@code edgesConnecting(nodeV, nodeU)}.
   *
   * <p>The resulting set of edges will be parallel (i.e. have equal {@link
   * #incidentNodes(Object)}). If this network does not {@link #allowsParallelEdges() allow parallel
   * edges}, the resulting set will contain at most one edge (equivalent to {@code
   * edgeConnecting(nodeU, nodeV).asSet()}).
   *

   *
   * <p>In an undirected network, this is equal to {@code edgesConnecting(nodeV, nodeU)}.
   *
   * <p>The resulting set of edges will be parallel (i.e. have equal {@link
   * #incidentNodes(Object)}). If this network does not {@link #allowsParallelEdges() allow parallel
   * edges}, the resulting set will contain at most one edge (equivalent to {@code
   * edgeConnecting(nodeU, nodeV).asSet()}).
   *

   *         <li>{@code inEdges(node)}: Edge insertion order
   *         <li>{@code outEdges(node)}: Edge insertion order
   *         <li>{@code adjacentEdges(edge)}: Stable order
   *         <li>{@code edgesConnecting(nodeU, nodeV)}: Edge insertion order
   *       </ul>
   * </ul>
   *
   * @since 29.0
   */
  public static <S> ElementOrder<S> stable() {
    return new ElementOrder<>(Type.STABLE, null);
  }

   *         <li>{@code inEdges(node)}: Edge insertion order
   *         <li>{@code outEdges(node)}: Edge insertion order
   *         <li>{@code adjacentEdges(edge)}: Stable order
   *         <li>{@code edgesConnecting(nodeU, nodeV)}: Edge insertion order
   *       </ul>
   * </ul>
   *
   * @since 29.0
   */
  public static <S> ElementOrder<S> stable() {
    return new ElementOrder<>(Type.STABLE, null);
  }