* AbstractSet#iterator()}.
 */
abstract class IncidentEdgeSet<N> extends AbstractSet<EndpointPair<N>> {
  final N node;
  final ArchetypeGraph<N> graph;
  final EdgeType edgeType;

  enum EdgeType {
    INCOMING, // incoming incident edges only
    OUTGOING, // outgoing incident edges only
    BOTH // both incoming and outgoing incident edges
  }

  IncidentEdgeSet(ArchetypeGraph<N> graph, N node, EdgeType edgeType) {

  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, IncidentEdgeSet.EdgeType.BOTH) {
          @Override
          public UnmodifiableIterator<EndpointPair<N>> iterator() {
            if (graph.isDirected()) {

  }

  @Override
  public Set<EndpointPair<N>> incidentEdges(N node) {
    GraphConnections<N, V> connections = checkedConnections(node);
    IncidentEdgeSet<N> incident =
        new IncidentEdgeSet<N>(this, node, IncidentEdgeSet.EdgeType.BOTH) {
          @Override
          public Iterator<EndpointPair<N>> iterator() {
            return connections.incidentEdgeIterator(node);
          }
        };

    public Set<N> successors(N node) {
      return delegate().predecessors(node); // transpose
    }

    @Override
    public Set<EndpointPair<N>> incidentEdges(N node) {
      return new IncidentEdgeSet<N>(this, node, IncidentEdgeSet.EdgeType.BOTH) {
        @Override
        public Iterator<EndpointPair<N>> iterator() {
          return Iterators.transform(
              delegate().incidentEdges(node).iterator(),