public static NetworkBuilder<Object, Object> directed() {
    return new NetworkBuilder<>(true);
  }

  /** Returns a {@link NetworkBuilder} for building undirected networks. */
  public static NetworkBuilder<Object, Object> undirected() {
    return new NetworkBuilder<>(false);
  }

  /**
   * Returns a {@link NetworkBuilder} initialized with all properties queryable from {@code

  }

  /**
   * Tests that the ImmutableNetwork.Builder doesn't change when the creating NetworkBuilder
   * changes.
   */
  @Test
  @SuppressWarnings("CheckReturnValue")
  public void immutableNetworkBuilder_copiesNetworkBuilder() {
    NetworkBuilder<String, Object> networkBuilder =
        NetworkBuilder.directed()
            .allowsSelfLoops(true)
            .<String>nodeOrder(ElementOrder.<String>natural());

  private static final NetworkBuilder<?, ?> NETWORK_BUILDER_A =
      NetworkBuilder.directed().allowsParallelEdges(true).expectedNodeCount(10);
  private static final NetworkBuilder<?, ?> NETWORK_BUILDER_B =
      NetworkBuilder.directed().allowsSelfLoops(true).expectedNodeCount(16);

  private static final ImmutableNetwork<String, String> IMMUTABLE_NETWORK_A =

  Network<Integer, String> createGraph() {
    networkBuilder =
        NetworkBuilder.directed()
            .allowsSelfLoops(allowsSelfLoops)
            .allowsParallelEdges(allowsParallelEdges)
            .nodeOrder(nodeOrder)
            .edgeOrder(edgeOrder)
            .immutable();

    return networkBuilder.build();
  }

  @Override
  void addNode(Integer n) {
    networkBuilder.addNode(n);

  @Test
  public void directedNetwork() {
    testNetworkMutation(NetworkBuilder.directed());
  }

  @Test
  public void undirectedNetwork() {
    testNetworkMutation(NetworkBuilder.undirected());
  }

  private static void testNetworkMutation(NetworkBuilder<? super Integer, Object> networkBuilder) {
    Random gen = new Random(42); // Fixed seed so test results are deterministic.

    MutableNetwork<Integer, String> network =
        NetworkBuilder.directed().edgeOrder(unordered()).build();

    assertThat(network.edgeOrder()).isEqualTo(unordered());
    assertThat(network.nodeOrder()).isEqualTo(insertion()); // default
  }

  @Test
  public void edgeOrder_insertion() {
    MutableNetwork<Integer, String> network =
        NetworkBuilder.directed().edgeOrder(insertion()).build();

  }

  /**
   * Tests that the ImmutableNetwork.Builder doesn't change when the creating NetworkBuilder
   * changes.
   */
  @Test
  @SuppressWarnings("CheckReturnValue")
  public void immutableNetworkBuilder_copiesNetworkBuilder() {
    NetworkBuilder<String, Object> networkBuilder =
        NetworkBuilder.directed()
            .allowsSelfLoops(true)
            .<String>nodeOrder(ElementOrder.<String>natural());

  private static final NetworkBuilder<?, ?> NETWORK_BUILDER_A =
      NetworkBuilder.directed().allowsParallelEdges(true).expectedNodeCount(10);
  private static final NetworkBuilder<?, ?> NETWORK_BUILDER_B =
      NetworkBuilder.directed().allowsSelfLoops(true).expectedNodeCount(16);

  private static final ImmutableNetwork<String, String> IMMUTABLE_NETWORK_A =

  Network<Integer, String> createGraph() {
    networkBuilder =
        NetworkBuilder.directed()
            .allowsSelfLoops(allowsSelfLoops)
            .allowsParallelEdges(allowsParallelEdges)
            .nodeOrder(nodeOrder)
            .edgeOrder(edgeOrder)
            .immutable();

    return networkBuilder.build();
  }

  @Override
  void addNode(Integer n) {
    networkBuilder.addNode(n);

    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 =
        NetworkBuilder.directed().allowsParallelEdges(true).allowsSelfLoops(true).build();