*
   * @throws IllegalStateException if {@code node} is already present
   */
  @CanIgnoreReturnValue
  private GraphConnections<N, V> addNodeInternal(N node) {
    GraphConnections<N, V> connections = newConnections();
    checkState(nodeConnections.put(node, connections) == null);
    return connections;
  }

  @Override
  @CanIgnoreReturnValue
  @CheckForNull
  public V putEdgeValue(N nodeU, N nodeV, V value) {

   *
   * @throws IllegalStateException if {@code node} is already present
   */
  @CanIgnoreReturnValue
  private NetworkConnections<N, E> addNodeInternal(N node) {
    NetworkConnections<N, E> connections = newConnections();
    checkState(nodeConnections.put(node, connections) == null);
    return connections;
  }

  @Override
  @CanIgnoreReturnValue
  public boolean addEdge(N nodeU, N nodeV, E edge) {