MutableValueGraph<Integer, String> undirectedGraph = ValueGraphBuilder.undirected().build();
    undirectedGraph.putEdgeValue(N1, N2, E12);

    assertThat(transpose(undirectedGraph)).isSameInstanceAs(undirectedGraph);
  }

  @Test
  public void transpose_directedValueGraph() {
    MutableValueGraph<Integer, String> directedGraph =
        ValueGraphBuilder.directed().allowsSelfLoops(true).build();

      ValueGraph<N, V> graph, Iterable<? extends N> nodes) {
    MutableValueGraph<N, V> subgraph =
        (nodes instanceof Collection)
            ? ValueGraphBuilder.from(graph).expectedNodeCount(((Collection) nodes).size()).build()
            : ValueGraphBuilder.from(graph).build();
    for (N node : nodes) {
      subgraph.addNode(node);
    }
    for (N node : subgraph.nodes()) {

      ValueGraph<N, V> graph, Iterable<? extends N> nodes) {
    MutableValueGraph<N, V> subgraph =
        (nodes instanceof Collection)
            ? ValueGraphBuilder.from(graph).expectedNodeCount(((Collection) nodes).size()).build()
            : ValueGraphBuilder.from(graph).build();
    for (N node : nodes) {
      subgraph.addNode(node);
    }
    for (N node : subgraph.nodes()) {

    MutableValueGraph<String, Integer> valueGraph = ValueGraphBuilder.directed().build();
    valueGraph.putEdgeValue("a", "b", 11);

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

  @Test
  public void forTree_withUndirectedValueGraph_throws() throws Exception {
    MutableValueGraph<String, Integer> valueGraph = ValueGraphBuilder.undirected().build();
    valueGraph.putEdgeValue("a", "b", 11);

    MutableValueGraph<String, Integer> valueGraph = ValueGraphBuilder.directed().build();
    valueGraph.putEdgeValue("a", "b", 11);

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

  @Test
  public void forTree_withUndirectedValueGraph_throws() throws Exception {
    MutableValueGraph<String, Integer> valueGraph = ValueGraphBuilder.undirected().build();
    valueGraph.putEdgeValue("a", "b", 11);