private final BaseGraph<N> graph;
  private final Iterator<N> nodeIterator;

  @CheckForNull
  N node = null; // null is safe as an initial value because graphs don't allow null nodes

  Iterator<N> successorIterator = ImmutableSet.<N>of().iterator();

  static <N> EndpointPairIterator<N> of(BaseGraph<N> graph) {

 * AbstractSet#iterator()}.
 */
@ElementTypesAreNonnullByDefault
abstract class IncidentEdgeSet<N> extends AbstractSet<EndpointPair<N>> {
  final N node;
  final BaseGraph<N> graph;

  IncidentEdgeSet(BaseGraph<N> graph, N node) {
    this.graph = graph;
    this.node = node;
  }

  @Override
  public boolean remove(@CheckForNull Object o) {
    throw new UnsupportedOperationException();
  }

 * AbstractSet#iterator()}.
 */
@ElementTypesAreNonnullByDefault
abstract class IncidentEdgeSet<N> extends AbstractSet<EndpointPair<N>> {
  final N node;
  final BaseGraph<N> graph;

  IncidentEdgeSet(BaseGraph<N> graph, N node) {
    this.graph = graph;
    this.node = node;
  }

  @Override
  public boolean remove(@CheckForNull Object o) {
    throw new UnsupportedOperationException();
  }

import java.util.Set;

/**
 * A class to allow {@link Graph} implementations to be backed by a {@link BaseGraph}. This is not
 * currently planned to be released as a general-purpose forwarding class.
 *
 * @author James Sexton
 */
@ElementTypesAreNonnullByDefault
abstract class ForwardingGraph<N> extends AbstractGraph<N> {

  abstract BaseGraph<N> delegate();

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

import java.util.Set;

/**
 * A class to allow {@link Graph} implementations to be backed by a {@link BaseGraph}. This is not
 * currently planned to be released as a general-purpose forwarding class.
 *
 * @author James Sexton
 */
@ElementTypesAreNonnullByDefault
abstract class ForwardingGraph<N> extends AbstractGraph<N> {

  abstract BaseGraph<N> delegate();

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

/**
 * This class provides a skeletal implementation of {@link BaseGraph}.
 *
 * <p>The methods implemented in this class should not be overridden unless the subclass admits a
 * more efficient implementation.
 *
 * @author James Sexton
 * @param <N> Node parameter type
 */
@ElementTypesAreNonnullByDefault
abstract class AbstractBaseGraph<N> implements BaseGraph<N> {

  /**

  StandardMutableGraph(AbstractGraphBuilder<? super N> builder) {
    this.backingValueGraph = new StandardMutableValueGraph<>(builder);
  }

  @Override
  BaseGraph<N> delegate() {
    return backingValueGraph;
  }

  @Override
  public boolean addNode(N node) {
    return backingValueGraph.addNode(node);
  }

  @Override
  public boolean putEdge(N nodeU, N nodeV) {

@Immutable(containerOf = {"N"})
@ElementTypesAreNonnullByDefault
public class ImmutableGraph<N> extends ForwardingGraph<N> {
  @SuppressWarnings("Immutable") // The backing graph must be immutable.
  private final BaseGraph<N> backingGraph;

  ImmutableGraph(BaseGraph<N> backingGraph) {
    this.backingGraph = backingGraph;
  }

  /** Returns an immutable copy of {@code graph}. */
  public static <N> ImmutableGraph<N> copyOf(Graph<N> graph) {