? DirectedGraphConnections.ofImmutable(
            node, graph.incidentEdges(node), successorNodeToValueFn)
        : UndirectedGraphConnections.ofImmutable(
            Maps.asMap(graph.adjacentNodes(node), successorNodeToValueFn));
  }

  /**
   * A builder for creating {@link ImmutableValueGraph} instances, especially {@code static final}
   * graphs. Example:
   *
   * <pre>{@code

    assertThat(graph.allowsSelfLoops()).isEqualTo(asGraph.allowsSelfLoops());

    for (Integer node : graph.nodes()) {
      assertThat(graph.adjacentNodes(node)).isEqualTo(asGraph.adjacentNodes(node));
      assertThat(graph.predecessors(node)).isEqualTo(asGraph.predecessors(node));
      assertThat(graph.successors(node)).isEqualTo(asGraph.successors(node));

   * returns an arbitrary (but consistent) endpoint of the origin edge.
   */
  public final N nodeU() {
    return nodeU;
  }

  /**
   * Returns the node {@link #adjacentNode(Object) adjacent} to {@link #nodeU()} along the origin
   * edge. If this {@link EndpointPair} {@link #isOrdered()}, this is equal to {@link #target()}.
   */
  public final N nodeV() {
    return nodeV;
  }

    }

    // requireNonNull is safe because of the edgeToReferenceNode check above.
    NetworkConnections<N, E> connectionsU = requireNonNull(nodeConnections.get(nodeU));
    N nodeV = connectionsU.adjacentNode(edge);
    NetworkConnections<N, E> connectionsV = requireNonNull(nodeConnections.get(nodeV));
    connectionsU.removeOutEdge(edge);
    connectionsV.removeInEdge(edge, allowsSelfLoops() && nodeU.equals(nodeV));

    assertThat(ordered.target()).isEqualTo("target");
    assertThat(ordered.nodeU()).isEqualTo("source");
    assertThat(ordered.nodeV()).isEqualTo("target");
    assertThat(ordered.adjacentNode("source")).isEqualTo("target");
    assertThat(ordered.adjacentNode("target")).isEqualTo("source");
    assertThat(ordered.toString()).isEqualTo("<source -> target>");
  }

  @Test
  public void testUnorderedEndpointPair() {

    assertThat(ordered.target()).isEqualTo("target");
    assertThat(ordered.nodeU()).isEqualTo("source");
    assertThat(ordered.nodeV()).isEqualTo("target");
    assertThat(ordered.adjacentNode("source")).isEqualTo("target");
    assertThat(ordered.adjacentNode("target")).isEqualTo("source");
    assertThat(ordered.toString()).isEqualTo("<source -> target>");
  }

  @Test
  public void testUnorderedEndpointPair() {

    return (E edge) -> network.incidentNodes(edge).target();
  }

  private static <N, E> Function<E, N> adjacentNodeFn(Network<N, E> network, N node) {
    return (E edge) -> network.incidentNodes(edge).adjacentNode(node);
  }

  /**
   * A builder for creating {@link ImmutableNetwork} instances, especially {@code static final}
   * networks. Example:
   *
   * <pre>{@code

  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.";
  static final String ENDPOINTS_MISMATCH =

    for (N node : nodes) {
      subgraph.addNode(node);
    }
    for (N node : subgraph.nodes()) {
      for (E edge : network.outEdges(node)) {
        N successorNode = network.incidentNodes(edge).adjacentNode(node);
        if (subgraph.nodes().contains(successorNode)) {
          subgraph.addEdge(node, successorNode, edge);
        }
      }
    }
    return subgraph;
  }