*     one path between any two nodes
   */
  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.");

 * @author Joshua O'Madadhain
 * @param <N> Node parameter type
 * @param <V> Value parameter type
 * @since 20.0
 */
@Beta
@ElementTypesAreNonnullByDefault
public interface ValueGraph<N, V> extends BaseGraph<N> {
  //
  // ValueGraph-level accessors
  //

  /** Returns all nodes in this graph, in the order specified by {@link #nodeOrder()}. */
  @Override
  Set<N> nodes();

 * @param <N> Node parameter type
 * @since 20.0
 */
@Beta
@DoNotMock("Use GraphBuilder to create a real instance")
@ElementTypesAreNonnullByDefault
public interface Graph<N> extends BaseGraph<N> {
  //
  // Graph-level accessors
  //

  /** Returns all nodes in this graph, in the order specified by {@link #nodeOrder()}. */
  @Override
  Set<N> nodes();