@Override
  public boolean processKeys(@NotNull Processor<? super K> processor) throws IOException {
    //TODO RC: forEachEntry() reads & deserializes both key and value -- but we don't need values here, only keys are needed.
    //         Specialize method so it reads only keys?
    return forEachEntry((key, value) -> {
      return processor.process(key);
    });
  }
@Override
  public boolean processKeys(@NotNull Processor<? super K> processor)