code: @SuppressWarnings("unchecked")
    ReservoirItemsSketch<T> copy() {
      return new ReservoirItemsSketch<>(reservoirSize_, currItemsAlloc_,
              itemsSeen_, rf_, (ArrayList<T>) data_.clone());
    }
docs: Used during union operations to ensure we do not overwrite an existing reservoir. Creates a
  shallow copy of the reservoir.
  
  @return A copy of the current sketch