    @Test
    public void testFlatten() {

        Octree<String> o = new Octree<>();
        o.insert(0, 0, 0, 8, "Foo");
        o.insert(32, 16, 8, 4, "Bar");

        List<String> f = o.flatten();
        assertEquals(2, f.size());
        assertTrue(f.contains("Foo"));
        assertTrue(f.contains("Bar"));
    }