* should prefer the {@link Graph} interface.
 *
 * @author James Sexton
 * @author Joshua O'Madadhain
 * @param <N> Node parameter type
 * @since 20.0
 */
public interface MutableGraph<N> extends Graph<N> {

  /**
   * Adds {@code node} if it is not already present.
   *
   * <p><b>Nodes must be unique</b>, just as {@code Map} keys must be. They must also be non-null.
   *

public final class ElementOrderTest {
  // Node order tests

  @Test
  public void nodeOrder_none() {
    MutableGraph<Integer> graph = GraphBuilder.directed().nodeOrder(unordered()).build();

    assertThat(graph.nodeOrder()).isEqualTo(unordered());
  }

  @Test
  public void nodeOrder_insertion() {
    MutableGraph<Integer> graph = GraphBuilder.directed().nodeOrder(insertion()).build();

    addNodes(graph);

   *
   * @since 28.0
   */
  public static class Builder<N> {

    private final MutableGraph<N> mutableGraph;

    Builder(GraphBuilder<N> graphBuilder) {
      // The incidentEdgeOrder for immutable graphs is always stable. However, we don't want to
      // modify this builder, so we make a copy instead.
      this.mutableGraph = graphBuilder.copy().incidentEdgeOrder(ElementOrder.<N>stable()).build();
    }

    /**

  }

  private static MutableGraph<Integer> buildDirectedGraph() {
    MutableGraph<Integer> directedGraph = GraphBuilder.directed().allowsSelfLoops(true).build();
    directedGraph.putEdge(N1, N1);
    directedGraph.putEdge(N1, N2);
    directedGraph.putEdge(N2, N1);

    return directedGraph;
  }

  private static MutableGraph<Integer> buildUndirectedGraph() {

  }

  // Tests for Graph.edges() and Network.asGraph().edges() methods
  // TODO(user): Move these to a more appropriate location in the test suite.

  @Test
  public void endpointPair_directedGraph() {
    MutableGraph<Integer> directedGraph = GraphBuilder.directed().allowsSelfLoops(true).build();
    directedGraph.addNode(N0);
    directedGraph.putEdge(N1, N2);
    directedGraph.putEdge(N2, N1);
    directedGraph.putEdge(N1, N3);

  }

  // Tests for Graph.edges() and Network.asGraph().edges() methods
  // TODO(user): Move these to a more appropriate location in the test suite.

  @Test
  public void endpointPair_directedGraph() {
    MutableGraph<Integer> directedGraph = GraphBuilder.directed().allowsSelfLoops(true).build();
    directedGraph.addNode(N0);
    directedGraph.putEdge(N1, N2);
    directedGraph.putEdge(N2, N1);
    directedGraph.putEdge(N1, N3);

    GraphBuilder<N1> newBuilder = cast();
    newBuilder.incidentEdgeOrder = checkNotNull(incidentEdgeOrder);
    return newBuilder;
  }

  /** Returns an empty {@link MutableGraph} with the properties of this {@link GraphBuilder}. */
  public <N1 extends N> MutableGraph<N1> build() {
    return new StandardMutableGraph<>(this);
  }

  GraphBuilder<N> copy() {
    GraphBuilder<N> newBuilder = new GraphBuilder<>(directed);

   *
   * @throws IllegalArgumentException if any element in {@code nodes} is not a node in the graph
   */
  public static <N> MutableGraph<N> inducedSubgraph(Graph<N> graph, Iterable<? extends N> nodes) {
    MutableGraph<N> subgraph =
        (nodes instanceof Collection)
            ? GraphBuilder.from(graph).expectedNodeCount(((Collection) nodes).size()).build()

  final boolean graphIsMutable() {
    return graphAsMutableGraph != null;
  }

  @Before
  public final void init() {
    graph = createGraph();
    if (graph instanceof MutableGraph) {
      graphAsMutableGraph = (MutableGraph<Integer>) graph;
    }
  }

  @After
  public final void validateGraphState() {
    validateGraph(graph);
  }

  static <N> void validateGraph(Graph<N> graph) {

 * create an instance of one of the built-in implementations of {@code Graph}, use the {@link
 * GraphBuilder} class:
 *
 * {@snippet :
 * MutableGraph<Integer> graph = GraphBuilder.undirected().build();
 * }
 *
 * <p>{@link GraphBuilder#build()} returns an instance of {@link MutableGraph}, which is a subtype
 * of {@code Graph} that provides methods for adding and removing nodes and edges. If you do not