code: public static <N> ImmutableGraph<N> singletonDirectedGraph(N node) {
          final MutableGraph<N> graph = GraphBuilder.directed().build();
          graph.addNode(node);
          return ImmutableGraph.copyOf(graph);
      }
docs: Returns an immutable directed graph, containing only the specified node.
  
  @param node The single node in the returned graph
  @param <N>  The class of the nodes
  @return an immutable directed graph with a single node