code: func (db *DB) SAdd(key []byte, args ...[]byte) (int64, error) {
  	t := db.setBatch
  	t.Lock()
  	defer t.Unlock()
  
  	var err error
  	var ek []byte
  	var num int64
  	for i := 0; i < len(args); i++ {
  		if err := checkSetKMSize(key, args[i]); err != nil {
  			return 0, err
  		}
  
  		ek = db.sEncodeSetKey(key, args[i])
  
  		if v, err := db.bucket.Get(ek); err != nil {
  			return 0, err
  		} else if v == nil {
  			num++
  		}
  
  		t.Put(ek, nil)
  	}
  
  	if _, err = db.sIncrSize(key, num); err != nil {
  		return 0, err
  	}
  
  	err = t.Commit()
  	return num, err
  
  }
docs: // SAdd adds the value to the set.