code: func (db *DB) ZScore(key []byte, member []byte) (int64, error) {
  	if err := checkZSetKMSize(key, member); err != nil {
  		return InvalidScore, err
  	}
  
  	score := InvalidScore
  
  	k := db.zEncodeSetKey(key, member)
  	if v, err := db.bucket.Get(k); err != nil {
  		return InvalidScore, err
  	} else if v == nil {
  		return InvalidScore, ErrScoreMiss
  	} else {
  		if score, err = Int64(v, nil); err != nil {
  			return InvalidScore, err
  		}
  	}
  
  	return score, nil
  }
docs: // ZScore gets the score of member.