code: public static <T, C extends Collection<T>> Reducer<T, C> getAggregatingReducer(
        Supplier<C> supplier) {
      return new AggregatingReducer<T, C>(supplier);
    }
docs: Gets a {@code Reducer} which aggregates items of type {@code T} into a
  collection of type {@code C}.
  
  @param supplier
  @return