}

  @Test
  public void successors_noSuccessors() {
    addNode(N1);
    assertThat(network.successors(N1)).isEmpty();
  }

  @Test
  public void successors_nodeNotInGraph() {
    assertNodeNotInGraphErrorMessage(
        assertThrows(IllegalArgumentException.class, () -> network.successors(NODE_NOT_IN_GRAPH)));
  }

  @Test
  public void addNode_newNode() {

  }

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

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

  }

  @Test
  public void successors_noSuccessors() {
    addNode(N1);
    assertThat(graph.successors(N1)).isEmpty();
  }

  @Test
  public void successors_nodeNotInGraph() {
    assertNodeNotInGraphErrorMessage(
        assertThrows(IllegalArgumentException.class, () -> graph.successors(NODE_NOT_IN_GRAPH)));
  }

  @Test

  @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;
  }

  }

  @Test
  public void successors_noSuccessors() {
    addNode(N1);
    assertThat(graph.successors(N1)).isEmpty();
  }

  @Test
  public void successors_nodeNotInGraph() {
    assertNodeNotInGraphErrorMessage(
        assertThrows(IllegalArgumentException.class, () -> graph.successors(NODE_NOT_IN_GRAPH)));
  }

  @Test

            if (next != null) {
              Iterator<? extends N> successors = successorFunction.successors(next).iterator();
              if (successors.hasNext()) {
                // BFS: horizon.addLast(successors)
                // Pre-order: horizon.addFirst(successors)
                order.insertInto(horizon, successors);
              }
              return next;
            }

  }

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

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

  }

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

  @Test

  }

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

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

 * which has a method {@code getChildren()} that retrieves its successors in a graph:
 *
 * <pre>{@code
 * someGraphAlgorithm(startNode, MyNode::getChildren);
 * }</pre>
 *
 * <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