@Override
        public void combine(final MapReduce.NullObject key, final Iterator<Long> values, final ReduceEmitter<MapReduce.NullObject, Long> emitter) {
            long counter = 0;
            while (values.hasNext()) {
                counter = counter + values.next();
            }
            emitter.emit(key, counter);
        }
@author Marko A. Rodriguez (http://markorodriguez.com)
spark-gremlin/src/test/java/org/apache/tinkerpop/gremlin/spark/process/computer/CombineIteratorTest.java