MutableGraph<Integer> directedGraph = GraphBuilder.directed().allowsSelfLoops(false).build();
    directedGraph.putEdge(N1, N2);
    directedGraph.putEdge(N1, N3);
    directedGraph.putEdge(N2, N3);
    directedGraph.addNode(N4);

    MutableGraph<Integer> expectedClosure = GraphBuilder.directed().allowsSelfLoops(true).build();
    expectedClosure.putEdge(N1, N1);

    private static final String MODULE_D = "module-d";
    private static final String MODULE_C_1 = "module-c-1";
    private static final String MODULE_C_2 = "module-c-2";

    private DefaultGraphBuilder graphBuilder;

    private final ProjectBuilder projectBuilder = mock(ProjectBuilder.class);
    private final MavenSession session = mock(MavenSession.class);

  public void immutableValueGraphBuilder_copiesGraphBuilder() {
    ValueGraphBuilder<String, Object> graphBuilder =
        ValueGraphBuilder.directed()
            .allowsSelfLoops(true)
            .<String>nodeOrder(ElementOrder.<String>natural());
    ImmutableValueGraph.Builder<String, Integer> immutableValueGraphBuilder =
        graphBuilder.<String, Integer>immutable();

  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");