* insertion order.
   */
  private void populateTShapedGraph() {
    putEdge(2, 1);
    putEdge(1, 4);
    putEdge(1, 3);
    putEdge(1, 2); // Duplicate
    putEdge(4, 5);
  }

  // Element Mutation

  @Test
  public void putEdge_existingNodes() {
    assume().that(graphIsMutable()).isTrue();

    // Adding nodes initially for safety (insulating from possible future
    // modifications to proxy methods)

   * insertion order.
   */
  private void populateTShapedGraph() {
    putEdge(2, 1);
    putEdge(1, 4);
    putEdge(1, 3);
    putEdge(1, 2); // Duplicate
    putEdge(4, 5);
  }

  // Element Mutation

  @Test
  public void putEdge_existingNodes() {
    assume().that(graphIsMutable()).isTrue();

    // Adding nodes initially for safety (insulating from possible future
    // modifications to proxy methods)

    addEdge(N1, N1, E11);
    assertThat(network.outDegree(N1)).isEqualTo(1);
    addEdge(N1, N2, E12);
    assertThat(network.outDegree(N1)).isEqualTo(2);
  }

  // Element Mutation

  @Test
  public void addEdge_existingNodes() {
    assume().that(graphIsMutable()).isTrue();

    // Adding nodes initially for safety (insulating from possible future
    // modifications to proxy methods)

    addEdge(N1, N1, E11);
    assertThat(network.outDegree(N1)).isEqualTo(1);
    addEdge(N1, N2, E12);
    assertThat(network.outDegree(N1)).isEqualTo(2);
  }

  // Element Mutation

  @Test
  public void addEdge_existingNodes() {
    assume().that(graphIsMutable()).isTrue();

    // Adding nodes initially for safety (insulating from possible future
    // modifications to proxy methods)

   */
  private void populateStarShapedGraph() {
    putEdge(2, 1);
    putEdge(1, 4);
    putEdge(1, 3);
    putEdge(5, 1);
    putEdge(1, 2);
    putEdge(3, 1);
  }

  // Element Mutation

  @Test
  public void putEdge_existingNodes() {
    assume().that(graphIsMutable()).isTrue();

    // Adding nodes initially for safety (insulating from possible future
    // modifications to proxy methods)

    addEdge(N1, N1, E11);
    assertThat(network.outDegree(N1)).isEqualTo(2);
    addEdge(N2, N1, E12);
    assertThat(network.outDegree(N1)).isEqualTo(3);
  }

  // Element Mutation

  @Test
  public void addEdge_existingNodes() {
    assume().that(graphIsMutable()).isTrue();

    // Adding nodes initially for safety (insulating from possible future
    // modifications to proxy methods)

   *       static factory method whose return type is {@code cls} or {@code cls}'s subtype.
   *   <li>Inequality check is not performed against state mutation methods such as {@link
   *       List#add}, or functional update methods such as {@link
   *       com.google.common.base.Joiner#skipNulls}.
   * </ul>
   *

    checkArgument(graph.nodes().contains(node), NODE_NOT_IN_GRAPH, node);
    return ImmutableSet.copyOf(Traverser.forGraph(graph).breadthFirst(node));
  }

  // Graph mutation methods

  // Graph view methods

  /**
   * Returns a view of {@code graph} with the direction (if any) of every edge reversed. All other

    checkArgument(graph.nodes().contains(node), NODE_NOT_IN_GRAPH, node);
    return ImmutableSet.copyOf(Traverser.forGraph(graph).breadthFirst(node));
  }

  // Graph mutation methods

  // Graph view methods

  /**
   * Returns a view of {@code graph} with the direction (if any) of every edge reversed. All other

   *       static factory method whose return type is {@code cls} or {@code cls}'s subtype.
   *   <li>Inequality check is not performed against state mutation methods such as {@link
   *       List#add}, or functional update methods such as {@link
   *       com.google.common.base.Joiner#skipNulls}.
   * </ul>
   *