Vertex c = graph.getVertex("c");
        Vertex d = graph.getVertex("d");
        assertEquals("a", a.getLabel());
        assertEquals("b", b.getLabel());
        assertEquals("c", c.getLabel());
        assertEquals("d", d.getLabel());
        assertFalse(hasEdge(graph, "b", "a"));
        assertFalse(hasEdge(graph, "a", "c"));
        assertFalse(hasEdge(graph, "a", "d"));