    @Test
    public void testVertices() throws Exception {

        int count = 0;
        Set<Object> ids = new HashSet<Object>();
        for (Vertex v : graph.getVertices()) {
            assertTrue(v instanceof MultiVertex);

            ids.add(v.getId());
            count++;
        }

        assertEquals(5, count);
        assertEquals(5, ids.size());
        assertTrue(ids.contains("Ford"));
        assertTrue(ids.contains("Arthur"));
        assertTrue(ids.contains("Zaphod"));
        assertTrue(ids.contains("Earth"));
        assertTrue(ids.contains("Betelgeuse"));
    }