code: func (txn *BytesTxn) Put(key string, data []byte) keyval.BytesTxn {
  	txn.access.Lock()
  	defer txn.access.Unlock()
  
  	txn.items[key] = &bytesTxnItem{data: data}
  
  	return txn
  }
docs: // Put adds store operation into transaction.