code: func (db *Database) Has(key []byte) (bool, error) {
  	db.lock.RLock()
  	defer db.lock.RUnlock()
  
  	if db.db == nil {
  		return false, errMemorydbClosed
  	}
  	_, ok := db.db[string(key)]
  	return ok, nil
  }
docs: // Has retrieves if a key is present in the key-value store.