public void immutableNetwork() {
    MutableNetwork<String, Integer> mutableNetwork = NetworkBuilder.directed().build();
    mutableNetwork.addNode("A");
    ImmutableNetwork<String, Integer> immutableNetwork = ImmutableNetwork.copyOf(mutableNetwork);

    assertThat(immutableNetwork).isNotInstanceOf(MutableNetwork.class);
    assertThat(immutableNetwork).isEqualTo(mutableNetwork);

    mutableNetwork.addNode("B");

  }

  @Test
  public void transpose_undirectedNetwork() {
    MutableNetwork<Integer, String> undirectedGraph = NetworkBuilder.undirected().build();
    undirectedGraph.addEdge(N1, N2, E12);

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

  @Test
  public void transpose_directedNetwork() {
    MutableNetwork<Integer, String> directedGraph =

 * create an instance of one of the built-in implementations of {@code Network}, use the {@link
 * NetworkBuilder} class:
 *
 * {@snippet :
 * MutableNetwork<Integer, MyEdge> network = NetworkBuilder.directed().build();
 * }
 *
 * <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();