@Test
public void testValidPublisher() {

    engine = new Engine();
    List<Stage> stages = new ArrayList<>();
    stages.add((Stage.Of) () -> Arrays.asList(1, 2, 3));
    stages.add((Stage.Map) () -> i -> (int) i + 1);
    Graph graph = () -> stages;
    assertThat(engine.buildPublisher(graph)).isNotNull();
}