@Test(expected = NullPointerException.class)
public void testWithoutEngine() {

    Graph g1 = () -> Collections.singletonList((Stage.Of) () -> Arrays.asList(1, 2, 3));
    Graph g2 = () -> Collections.singletonList((Stage.Of) () -> Arrays.asList(1, 2, 3));
    factory.create(null, new Stage.Concat() {
        @Override
        public Graph getFirst() {
            return g1;
        }
        @Override
        public Graph getSecond() {
            return g2;
        }
    });
}