@Override
        public Map<E, Long> apply(final Map<E, Long> mutatingSeed, final Map<E, Long> map) {
            for (final Map.Entry<E, Long> entry : map.entrySet()) {
                MapHelper.incr(mutatingSeed, entry.getKey(), entry.getValue());
            }
            return mutatingSeed;
        }
@author Marko A. Rodriguez (http://markorodriguez.com)
gremlin-core/src/main/java/org/apache/tinkerpop/gremlin/process/traversal/step/map/GroupCountStep.java