public static <N> ImmutableGraph<N> singletonDirectedGraph(N node) {
        final MutableGraph<N> graph = GraphBuilder.directed().build();
        graph.addNode(node);
        return ImmutableGraph.copyOf(graph);
    }
@Test
    public void singletonDirectedGraph() {
        final ImmutableGraph<String> singletonGraph = Graphs.singletonDirectedGraph("Test");
        assertThat(singletonGraph.isDirected()).isTrue();
        assertThat(singletonGraph.nodes()).containsExactly("Test");
        assertThat(singletonGraph.edges()).isEmpty();
    }