@Override
  public Set<N> successors() {
    return Collections.unmodifiableSet(successorsMultiset().elementSet());
  }

  private Multiset<N> successorsMultiset() {
    Multiset<N> successors = getReference(successorsReference);
    if (successors == null) {
      successors = HashMultiset.create(outEdgeMap.values());
      successorsReference = new SoftReference<>(successors);
    }
    return successors;
  }

  @Override
  public Set<N> successors() {
    return Collections.unmodifiableSet(successorsMultiset().elementSet());
  }

  private Multiset<N> successorsMultiset() {
    Multiset<N> successors = getReference(successorsReference);
    if (successors == null) {
      successors = HashMultiset.create(outEdgeMap.values());
      successorsReference = new SoftReference<>(successors);
    }
    return successors;
  }

                      Iterators.transform(
                          // filter out 'node' from successors (already covered by predecessors,
                          // above)
                          Sets.difference(graph.successors(node), ImmutableSet.of(node)).iterator(),
                          (N successor) -> EndpointPair.ordered(node, successor))));
            } else {

    assume().that(graphIsMutable()).isTrue();

    addNode(N1);
    Set<Integer> successors = graph.successors(N1);
    assertThrows(UnsupportedOperationException.class, () -> successors.add(N2));
    putEdge(N1, N2);
    assertThat(successors).containsExactlyElementsIn(graph.successors(N1));
  }

  @Override
  @Test
  public void incidentEdges_checkReturnedSetMutability() {

    assume().that(graphIsMutable()).isTrue();

    addNode(N1);
    Set<Integer> successors = graph.successors(N1);
    assertThrows(UnsupportedOperationException.class, () -> successors.add(N2));
    putEdge(N1, N2);
    assertThat(graph.successors(N1)).containsExactlyElementsIn(successors);
  }

  @Override
  @Test
  public void incidentEdges_checkReturnedSetMutability() {

    assume().that(graphIsMutable()).isTrue();

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

  @Test
  public void edges_containsOrderMismatch() {
    addEdge(N1, N2, E12);

    assume().that(graphIsMutable()).isTrue();

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

  @Test
  public void edges_containsOrderMismatch() {
    addEdge(N1, N2, E12);

    return delegate().adjacentNodes(node);
  }

  @Override
  public Set<N> predecessors(N node) {
    return delegate().predecessors(node);
  }

  @Override
  public Set<N> successors(N node) {
    return delegate().successors(node);
  }

  @Override
  public int degree(N node) {
    return delegate().degree(node);
  }

  @Override
  public int inDegree(N node) {
    return delegate().inDegree(node);

 * which has a method {@code getChildren()} that retrieves its successors in a graph:
 *
 * {@snippet :
 * someGraphAlgorithm(startNode, MyNode::getChildren);
 * }
 *
 * <p>If you have some other mechanism for returning the successors of a node, or one that doesn't
 * return an {@code Iterable<? extends N>}, then you can use a lambda to perform a more general

  @Override
  @Test
  public void successors_checkReturnedSetMutability() {
    addNode(N1);
    Set<Integer> successors = network.successors(N1);
    assertThrows(UnsupportedOperationException.class, () -> successors.add(N2));
    addEdge(N1, N2, E12);
    assertThat(network.successors(N1)).containsExactlyElementsIn(successors);
  }

  @Test
  public void edges_containsOrderMismatch() {
    addEdge(N1, N2, E12);