}

  private final GraphConnections<N, V> checkedConnections(N node) {
    GraphConnections<N, V> connections = nodeConnections.get(node);
    if (connections == null) {
      checkNotNull(node);
      throw new IllegalArgumentException("Node " + node + " is not an element of this graph.");
    }
    return connections;
  }

  final boolean containsNode(@CheckForNull N node) {
    return nodeConnections.containsKey(node);

  /** The HTTP {@code Authorization} header field name. */
  public static final String AUTHORIZATION = "Authorization";
  /** The HTTP {@code Connection} header field name. */
  public static final String CONNECTION = "Connection";
  /** The HTTP {@code Cookie} header field name. */
  public static final String COOKIE = "Cookie";
  /**

  }

  final NetworkConnections<N, E> checkedConnections(N node) {
    NetworkConnections<N, E> connections = nodeConnections.get(node);
    if (connections == null) {
      checkNotNull(node);
      throw new IllegalArgumentException(String.format(NODE_NOT_IN_GRAPH, node));
    }
    return connections;
  }

  final N checkedReferenceNode(E edge) {
    N referenceNode = edgeToReferenceNode.get(edge);