}

  @Override
  public N adjacentNode(E edge) {
    // Since the reference node is defined to be 'source' for directed graphs,
    // we can assume this edge lives in the set of outgoing edges.
    // (We're relying on callers to call this method only with an edge that's in the graph.)
    return requireNonNull(outEdgeMap.get(edge));
  }

  @Override
  public N removeInEdge(E edge, boolean isSelfLoop) {
    if (isSelfLoop) {

  }

  @Override
  @Test
  public void edges_checkReturnedSetMutability() {
    Set<String> edges = network.edges();
    UnsupportedOperationException e =
        assertThrows(UnsupportedOperationException.class, () -> edges.add(E12));
    addEdge(N1, N2, E12);
    assertThat(network.edges()).containsExactlyElementsIn(edges);
  }

  @Override
  @Test

   * treated as if collapsed into a single edge. For example, the {@link #degree(Object)} of a node
   * in the {@link Graph} view may be less than the degree of the same node in this {@link Network}.
   */
  Graph<N> asGraph();

  //
  // Network properties
  //

  /**
   * Returns true if the edges in this network are directed. Directed edges connect a {@link

  @Override
  Set<N> nodes();

  /** Returns all edges in this graph. */
  @Override
  Set<EndpointPair<N>> edges();

  //
  // Graph properties
  //

  /**
   * Returns true if the edges in this graph are directed. Directed edges connect a {@link
   * EndpointPair#source() source node} to a {@link EndpointPair#target() target node}, while
   * undirected edges connect a pair of nodes to each other.
   */

  Set<N> nodes();

  /** Returns all edges in this graph. */
  Set<EndpointPair<N>> edges();

  //
  // Graph properties
  //

  /**
   * Returns true if the edges in this graph are directed. Directed edges connect a {@link
   * EndpointPair#source() source node} to a {@link EndpointPair#target() target node}, while
   * undirected edges connect a pair of nodes to each other.
   */
  boolean isDirected();

  }

  @Test
  public void edges_oneEdge() {
    addEdge(N1, N2, E12);
    assertThat(network.edges()).containsExactly(E12);
  }

  @Test
  public void edges_noEdges() {
    assertThat(network.edges()).isEmpty();
    // Network with no edges, given disconnected nodes
    addNode(N1);
    addNode(N2);
    assertThat(network.edges()).isEmpty();
  }

  @Test

  }

  @Test
  public void edges_oneEdge() {
    addEdge(N1, N2, E12);
    assertThat(network.edges()).containsExactly(E12);
  }

  @Test
  public void edges_noEdges() {
    assertThat(network.edges()).isEmpty();
    // Network with no edges, given disconnected nodes
    addNode(N1);
    addNode(N2);
    assertThat(network.edges()).isEmpty();
  }

  @Test

    GraphBuilder<N1> castBuilder = cast();
    return new ImmutableGraph.Builder<>(castBuilder);
  }

  /**
   * Specifies whether the graph will allow self-loops (edges that connect a node to itself).
   * Attempting to add a self-loop to a graph that does not allow them will throw an {@link
   * UnsupportedOperationException}.
   *
   * <p>The default value is {@code false}.
   */

      assertThat(graph.nodes()).hasSize(NUM_NODES);
      assertThat(graph.edges()).hasSize(NUM_EDGES);
      AbstractGraphTest.validateGraph(graph);

      Collections.shuffle(edgeList, gen);
      int numEdgesToRemove = gen.nextInt(NUM_EDGES);
      for (int i = 0; i < numEdgesToRemove; ++i) {
        EndpointPair<Integer> edge = edgeList.get(i);

  }

  @Override
  @Test
  public void edges_checkReturnedSetMutability() {
    Set<String> edges = network.edges();
    UnsupportedOperationException e =
        assertThrows(UnsupportedOperationException.class, () -> edges.add(E12));
    addEdge(N1, N2, E12);
    assertThat(network.edges()).containsExactlyElementsIn(edges);
  }

  @Override
  @Test