*
   * <ul>
   *   <li>A and B have equal {@link #isDirected() directedness}.
   *   <li>A and B have equal {@link #nodes() node sets}.
   *   <li>A and B have equal {@link #edges() edge sets}.
   *   <li>Every edge in A and B connects the same nodes in the same direction (if any).
   * </ul>
   *
   * <p>Network properties besides {@link #isDirected() directedness} do <b>not</b> affect equality.

   *
   * <ul>
   *   <li>A and B have equal {@link #isDirected() directedness}.
   *   <li>A and B have equal {@link #nodes() node sets}.
   *   <li>A and B have equal {@link #edges() edge sets}.
   *   <li>Every edge in A and B connects the same nodes in the same direction (if any).
   * </ul>
   *
   * <p>Network properties besides {@link #isDirected() directedness} do <b>not</b> affect equality.

  private static <N> GraphConnections<N, Presence> connectionsOf(Graph<N> graph, N node) {
    Function<N, Presence> edgeValueFn =
        (Function<N, Presence>) Functions.constant(Presence.EDGE_EXISTS);
    return graph.isDirected()
        ? DirectedGraphConnections.ofImmutable(node, graph.incidentEdges(node), edgeValueFn)
        : UndirectedGraphConnections.ofImmutable(
            Maps.asMap(graph.adjacentNodes(node), edgeValueFn));
  }

  private static <N> GraphConnections<N, Presence> connectionsOf(Graph<N> graph, N node) {
    Function<N, Presence> edgeValueFn =
        (Function<N, Presence>) Functions.constant(Presence.EDGE_EXISTS);
    return graph.isDirected()
        ? DirectedGraphConnections.ofImmutable(node, graph.incidentEdges(node), edgeValueFn)
        : UndirectedGraphConnections.ofImmutable(
            Maps.asMap(graph.adjacentNodes(node), edgeValueFn));
  }

        (N successorNode) ->
            // requireNonNull is safe because the endpoint pair comes from the graph.
            requireNonNull(graph.edgeValueOrDefault(node, successorNode, null));
    return graph.isDirected()
        ? DirectedGraphConnections.ofImmutable(
            node, graph.incidentEdges(node), successorNodeToValueFn)
        : UndirectedGraphConnections.ofImmutable(

        (N successorNode) ->
            // requireNonNull is safe because the endpoint pair comes from the graph.
            requireNonNull(graph.edgeValueOrDefault(node, successorNode, null));
    return graph.isDirected()
        ? DirectedGraphConnections.ofImmutable(
            node, graph.incidentEdges(node), successorNodeToValueFn)
        : UndirectedGraphConnections.ofImmutable(

  public static <N> Traverser<N> forTree(SuccessorsFunction<N> tree) {
    if (tree instanceof BaseGraph) {
      checkArgument(((BaseGraph<?>) tree).isDirected(), "Undirected graphs can never be trees.");
    }
    if (tree instanceof Network) {
      checkArgument(((Network<?, ?>) tree).isDirected(), "Undirected networks can never be trees.");
    }
    return new Traverser<N>(tree) {
      @Override
      Traversal<N> newTraversal() {

  public static <N> Traverser<N> forTree(SuccessorsFunction<N> tree) {
    if (tree instanceof BaseGraph) {
      checkArgument(((BaseGraph<?>) tree).isDirected(), "Undirected graphs can never be trees.");
    }
    if (tree instanceof Network) {
      checkArgument(((Network<?, ?>) tree).isDirected(), "Undirected networks can never be trees.");
    }
    return new Traverser<N>(tree) {
      @Override
      Traversal<N> newTraversal() {

   * EndpointPair#source() source node} to a {@link EndpointPair#target() target node}, while
   * undirected edges connect a pair of nodes to each other.
   */
  boolean isDirected();

  /**
   * Returns true if this graph allows self-loops (edges that connect a node to itself). Attempting
   * to add a self-loop to a graph that does not allow them will throw an {@link
   * IllegalArgumentException}.

   * EndpointPair#source() source node} to a {@link EndpointPair#target() target node}, while
   * undirected edges connect a pair of nodes to each other.
   */
  boolean isDirected();

  /**
   * Returns true if this graph allows self-loops (edges that connect a node to itself). Attempting
   * to add a self-loop to a graph that does not allow them will throw an {@link
   * IllegalArgumentException}.