* subtype of {@code ValueGraph} that provides methods for adding and removing nodes and edges. If
 * you do not need to mutate a graph (e.g. if you write a method than runs a read-only algorithm on
 * the graph), you should use the non-mutating {@link ValueGraph} interface, or an {@link
 * ImmutableValueGraph}.
 *
 * <p>You can create an immutable copy of an existing {@code ValueGraph} using {@link

    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);

  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);

      public Graph<N> asGraph() {
        if (AbstractBaseGraph.this instanceof Graph) {
          return (Graph<N>) AbstractBaseGraph.this;
        } else if (AbstractBaseGraph.this instanceof ValueGraph) {
          return ((ValueGraph<N, ?>) AbstractBaseGraph.this).asGraph();
        }
        throw new UnsupportedOperationException(
            "Unexpected graph type: " + AbstractBaseGraph.this.getClass());
      }

 * <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">

 * <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">

 * <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">