code: func (r *ProtoKeyValWrapper) GetPrevValue(prevValue proto.Message) (prevValueExist bool, err error) {
  	exists, err := r.ProtoKeyVal.GetPrevValue(prevValue)
  	if !exists || err != nil {
  		return exists, err
  	}
  	_, err = r.decrypter.Decrypt(prevValue, r.decryptFunc)
  	return exists, err
  }
docs: // GetPrevValue returns the previous value of the pair.