@Override
        public void combine(final NullObject key, final Iterator<Integer> values, final ReduceEmitter<NullObject, Integer> emitter) {
            emitter.emit(2);
            assertEquals(2, countTrue(WORKER_START));
            assertTrue(WORKER_START.contains(Stage.MAP) && WORKER_START.contains(Stage.COMBINE));
            assertFalse(WORKER_END.isEmpty());
        }
@author Marko A. Rodriguez (http://markorodriguez.com)
gremlin-test/src/main/java/org/apache/tinkerpop/gremlin/process/computer/GraphComputerTest.java