* NetworkBuilder} class:
 *
 * <pre>{@code
 * MutableNetwork<Integer, MyEdge> network = NetworkBuilder.directed().build();
 * }</pre>
 *
 * <p>{@link NetworkBuilder#build()} returns an instance of {@link MutableNetwork}, which is a
 * subtype of {@code Network} that provides methods for adding and removing nodes and edges. If you

  public void forTree_acceptsDirectedNetwork() throws Exception {
    MutableNetwork<String, Integer> network = NetworkBuilder.directed().build();
    network.addEdge("a", "b", 11);

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

  @Test
  public void forTree_withUndirectedNetwork_throws() throws Exception {
    MutableNetwork<String, Integer> network = NetworkBuilder.undirected().build();

  public void forTree_acceptsDirectedNetwork() throws Exception {
    MutableNetwork<String, Integer> network = NetworkBuilder.directed().build();
    network.addEdge("a", "b", 11);

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

  @Test
  public void forTree_withUndirectedNetwork_throws() throws Exception {
    MutableNetwork<String, Integer> network = NetworkBuilder.undirected().build();