public String sprites() {
        return get(SPRITES, null);
    }
@Test
    public void setSprites() {
        loadLayout(L3);
        assertEquals("sprites not westminster", UK_LONDON_WEST, cfg.sprites());
        cfg.sprites(NEW_SPR);
        assertEquals("not new sprites", NEW_SPR, cfg.sprites());
        cfg.sprites(null);
        assertNull("sprites not cleared", cfg.sprites());
    }