}

  private static MutableGraph<Integer> buildDirectedGraph() {
    MutableGraph<Integer> directedGraph = GraphBuilder.directed().allowsSelfLoops(true).build();
    directedGraph.putEdge(N1, N1);
    directedGraph.putEdge(N1, N2);
    directedGraph.putEdge(N2, N1);

    return directedGraph;
  }

  private static MutableGraph<Integer> buildUndirectedGraph() {

   *
   * @throws IllegalArgumentException if any element in {@code nodes} is not a node in the graph
   */
  public static <N> MutableGraph<N> inducedSubgraph(Graph<N> graph, Iterable<? extends N> nodes) {
    MutableGraph<N> subgraph =
        (nodes instanceof Collection)
            ? GraphBuilder.from(graph).expectedNodeCount(((Collection) nodes).size()).build()

   *
   * @throws IllegalArgumentException if any element in {@code nodes} is not a node in the graph
   */
  public static <N> MutableGraph<N> inducedSubgraph(Graph<N> graph, Iterable<? extends N> nodes) {
    MutableGraph<N> subgraph =
        (nodes instanceof Collection)
            ? GraphBuilder.from(graph).expectedNodeCount(((Collection) nodes).size()).build()

  public void forTree_acceptsDirectedGraph() throws Exception {
    MutableGraph<String> graph = GraphBuilder.directed().build();
    graph.putEdge("a", "b");

    Traverser.forTree(graph); // Does not throw
  }

  @Test
  public void forTree_withUndirectedGraph_throws() throws Exception {
    MutableGraph<String> graph = GraphBuilder.undirected().build();
    graph.putEdge("a", "b");

  public void forTree_acceptsDirectedGraph() throws Exception {
    MutableGraph<String> graph = GraphBuilder.directed().build();
    graph.putEdge("a", "b");

    Traverser.forTree(graph); // Does not throw
  }

  @Test
  public void forTree_withUndirectedGraph_throws() throws Exception {
    MutableGraph<String> graph = GraphBuilder.undirected().build();
    graph.putEdge("a", "b");