code: func (db *DB) SIsMember(key []byte, member []byte) (int64, error) {
  	ek := db.sEncodeSetKey(key, member)
  
  	var n int64 = 1
  	if v, err := db.bucket.Get(ek); err != nil {
  		return 0, err
  	} else if v == nil {
  		n = 0
  	}
  	return n, nil
  }
docs: // SIsMember checks member in set.