/**
 * Abstract base class for an incident edges set that allows different implementations of {@link
 * AbstractSet#iterator()}.
 */
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(@Nullable Object o) {

  @Override
  public Set<EndpointPair<N>> incidentEdges(N node) {
    checkNotNull(node);
    checkArgument(nodes().contains(node), "Node %s is not an element of this graph.", node);
    IncidentEdgeSet<N> incident =
        new IncidentEdgeSet<N>(this, node) {
          @Override
          public UnmodifiableIterator<EndpointPair<N>> iterator() {
            if (graph.isDirected()) {
              return Iterators.unmodifiableIterator(