code: public static <K, V> MutableMultimap<K, V> groupBy(
              Iterable<V> iterable,
              Function<? super V, ? extends K> function,
              int batchSize,
              ForkJoinPool executor)
      {
          return FJIterate.groupBy(iterable, function, SynchronizedPutFastListMultimap.<K, V>newMultimap(), batchSize, executor);
      }
docs: Same effect as {@link Iterate#groupBy(Iterable, Function)},
  but executed in parallel batches, and writing output into a SynchronizedPutFastListMultimap.