@Immutable(containerOf = {"N"})
public abstract class EndpointPair<N> implements Iterable<N> {
  private final N nodeU;
  private final N nodeV;

  private EndpointPair(N nodeU, N nodeV) {
    this.nodeU = checkNotNull(nodeU);
    this.nodeV = checkNotNull(nodeV);
  }

  /** Returns an {@link EndpointPair} representing the endpoints of a directed edge. */
  public static <N> EndpointPair<N> ordered(N source, N target) {

          // operations only in weird cases like checking for an EndpointPair<ArrayList> in a
          // Network<LinkedList>.
          @SuppressWarnings("unchecked")
          @Override
          public boolean contains(@Nullable Object obj) {
            if (!(obj instanceof EndpointPair)) {
              return false;
            }
            EndpointPair<?> endpointPair = (EndpointPair<?>) obj;
            return isOrderingCompatible(endpointPair)

    public Set<E> outEdges(N node) {
      return delegate().inEdges(node); // transpose
    }

    @Override
    public EndpointPair<N> incidentNodes(E edge) {
      EndpointPair<N> endpointPair = delegate().incidentNodes(edge);
      return EndpointPair.of(network, endpointPair.nodeV(), endpointPair.nodeU()); // transpose
    }

    @Override
    public Set<E> edgesConnecting(N nodeU, N nodeV) {

  }

  @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);
          }

  Graph<N> asGraph();

  //
  // ValueGraph properties
  //

  /**
   * Returns true if the edges in this graph are directed. Directed edges connect a {@link
   * EndpointPair#source() source node} to a {@link EndpointPair#target() target node}, while
   * undirected edges connect a pair of nodes to each other.
   */
  @Override
  boolean isDirected();

  /**

   * any, specified by {@code endpoints}).
   *
   * <p>Unlike the other {@code EndpointPair}-accepting methods, this method does not throw if the
   * endpoints are unordered and the network is directed; it simply returns {@code false}. This is
   * for consistency with {@link Graph#hasEdgeConnecting(EndpointPair)} and {@link
   * ValueGraph#hasEdgeConnecting(EndpointPair)}.
   *
   * @since 27.1
   */
  @Override

    assertThat(graph.hasEdgeConnecting(EndpointPair.unordered(1, 2))).isFalse();
    assertThat(graph.hasEdgeConnecting(EndpointPair.unordered(2, 1))).isFalse();
  }

  @Test
  public void hasEdgeConnecting_undirected_correct() {
    graph = ValueGraphBuilder.undirected().build();
    graph.putEdgeValue(1, 2, "A");
    assertThat(graph.hasEdgeConnecting(EndpointPair.unordered(1, 2))).isTrue();
  }

  @Test

  public Graph<N> asGraph() {
    return new AbstractGraph<N>() {
      @Override
      public Set<N> nodes() {
        return AbstractValueGraph.this.nodes();
      }

      @Override
      public Set<EndpointPair<N>> edges() {
        return AbstractValueGraph.this.edges();
      }

      @Override
      public boolean isDirected() {
        return AbstractValueGraph.this.isDirected();
      }

      @Override

   *     #allowsSelfLoops()}
   * @throws IllegalArgumentException if the endpoints are unordered and the graph is directed
   * @since 27.1
   */
  @CanIgnoreReturnValue
  boolean putEdge(EndpointPair<N> endpoints);

  /**
   * Removes {@code node} if it is present; all edges incident to {@code node} will also be removed.
   *
   * @return {@code true} if the graph was modified as a result of this call
   */

  @Override
  Set<N> nodes();

  /** Returns all edges in this graph. */
  @Override
  Set<EndpointPair<N>> edges();

  //
  // Graph properties
  //

  /**
   * Returns true if the edges in this graph are directed. Directed edges connect a {@link
   * EndpointPair#source() source node} to a {@link EndpointPair#target() target node}, while
   * undirected edges connect a pair of nodes to each other.
   */