private static final class TransposedValueGraph<N, V> extends ForwardingValueGraph<N, V> {
    private final ValueGraph<N, V> graph;

    TransposedValueGraph(ValueGraph<N, V> graph) {
      this.graph = graph;
    }

    @Override
    ValueGraph<N, V> delegate() {
      return graph;
    }

    @Override
    public Set<N> predecessors(N node) {

  private static final class TransposedValueGraph<N, V> extends ForwardingValueGraph<N, V> {
    private final ValueGraph<N, V> graph;

    TransposedValueGraph(ValueGraph<N, V> graph) {
      this.graph = graph;
    }

    @Override
    ValueGraph<N, V> delegate() {
      return graph;
    }

    @Override
    public Set<N> predecessors(N node) {

  public void copyOf_directedValueGraph() {
    ValueGraph<Integer, String> directedGraph = buildDirectedValueGraph();

    ValueGraph<Integer, String> copy = copyOf(directedGraph);
    assertThat(copy).isEqualTo(directedGraph);
  }

  @Test
  public void copyOf_undirectedValueGraph() {
    ValueGraph<Integer, String> undirectedGraph = buildUndirectedValueGraph();

    ValueGraph<Integer, String> copy = copyOf(undirectedGraph);

    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);

 * <p>A graph is composed of a set of nodes and a set of edges connecting pairs of nodes.
 *
 * <p>There are three primary interfaces provided to represent graphs. In order of increasing
 * complexity they are: {@link Graph}, {@link ValueGraph}, and {@link Network}. You should generally
 * prefer the simplest interface that satisfies your use case. See the <a
 * href="https://github.com/google/guava/wiki/GraphsExplained#choosing-the-right-graph-type">