}

  @Override
  public EndpointPair<N> incidentNodes(E edge) {
    N nodeU = checkedReferenceNode(edge);
    // requireNonNull is safe because checkedReferenceNode made sure the edge is in the network.
    N nodeV = requireNonNull(nodeConnections.get(nodeU)).adjacentNode(edge);
    return EndpointPair.of(this, nodeU, nodeV);
  }

  @Override

  }

  @Override
  public EndpointPair<N> incidentNodes(E edge) {
    N nodeU = checkedReferenceNode(edge);
    // requireNonNull is safe because checkedReferenceNode made sure the edge is in the network.
    N nodeV = requireNonNull(nodeConnections.get(nodeU)).adjacentNode(edge);
    return EndpointPair.of(this, nodeU, nodeV);
  }

  @Override

          + "that allows self-loops, call allowsSelfLoops(true) on the Builder.";
  static final String NOT_AVAILABLE_ON_UNDIRECTED =
      "Cannot call source()/target() on a EndpointPair from an undirected graph. Consider calling "
          + "adjacentNode(node) if you already have a node, or nodeU()/nodeV() if you don't.";
  static final String EDGE_ALREADY_EXISTS = "Edge %s already exists in the graph.";

   *     #allowsParallelEdges()} or {@link #allowsSelfLoops()}
   * @throws IllegalArgumentException if the endpoints are unordered and the graph is directed
   * @since 27.1
   */
  @CanIgnoreReturnValue
  boolean addEdge(EndpointPair<N> endpoints, E edge);

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

   *     #allowsParallelEdges()} or {@link #allowsSelfLoops()}
   * @throws IllegalArgumentException if the endpoints are unordered and the graph is directed
   * @since 27.1
   */
  @CanIgnoreReturnValue
  boolean addEdge(EndpointPair<N> endpoints, E edge);

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

     *     #allowsSelfLoops()}
     * @throws IllegalArgumentException if the endpoints are unordered and the graph is directed
     */
    @CanIgnoreReturnValue
    public Builder<N> putEdge(EndpointPair<N> endpoints) {
      mutableGraph.putEdge(endpoints);
      return this;
    }

    /**
     * Returns a newly-created {@code ImmutableGraph} based on the contents of this {@code Builder}.
     */

     *     #allowsSelfLoops()}
     * @throws IllegalArgumentException if the endpoints are unordered and the graph is directed
     */
    @CanIgnoreReturnValue
    public ImmutableValueGraph.Builder<N, V> putEdgeValue(EndpointPair<N> endpoints, V value) {
      mutableValueGraph.putEdgeValue(endpoints, value);
      return this;
    }

    /**
     * Returns a newly-created {@code ImmutableValueGraph} based on the contents of this {@code

     *     #allowsSelfLoops()}
     * @throws IllegalArgumentException if the endpoints are unordered and the graph is directed
     */
    @CanIgnoreReturnValue
    public ImmutableValueGraph.Builder<N, V> putEdgeValue(EndpointPair<N> endpoints, V value) {
      mutableValueGraph.putEdgeValue(endpoints, value);
      return this;
    }

    /**
     * Returns a newly-created {@code ImmutableValueGraph} based on the contents of this {@code