code: func (s *BufferStore) Iter(k Key, upperBound Key) (Iterator, error) {
  	bufferIt, err := s.MemBuffer.Iter(k, upperBound)
  	if err != nil {
  		return nil, err
  	}
  	retrieverIt, err := s.r.Iter(k, upperBound)
  	if err != nil {
  		return nil, err
  	}
  	return NewUnionIter(bufferIt, retrieverIt, false)
  }
docs: // Iter implements the Retriever interface.