@SuppressWarnings("unchecked")
    public static <N> ImmutableGraph<N> emptyDirectedGraph() {
        return EMPTY_DIRECTED_GRAPH;
    }
@Test
    public void emptyDirectedGraph() {
        final ImmutableGraph<String> emptyGraph = Graphs.emptyDirectedGraph();
        assertThat(emptyGraph.isDirected()).isTrue();
        assertThat(emptyGraph.nodes()).isEmpty();
        assertThat(emptyGraph.edges()).isEmpty();
    }