code: @CanIgnoreReturnValue
    private GraphConnections<N, V> addNodeInternal(N node) {
      GraphConnections<N, V> connections = newConnections();
      checkState(nodeConnections.put(node, connections) == null);
      return connections;
    }
docs: Adds {@code node} to the graph and returns the associated {@link GraphConnections}.
  
  @throws IllegalStateException if {@code node} is already present