code: func (s *BufferStore) Get(k Key) ([]byte, error) {
  	val, err := s.MemBuffer.Get(k)
  	if IsErrNotFound(err) {
  		val, err = s.r.Get(k)
  	}
  	if err != nil {
  		return nil, err
  	}
  	if len(val) == 0 {
  		return nil, ErrNotExist
  	}
  	return val, nil
  }
docs: // Get implements the Retriever interface.