code: func (tx *bytesTxn) Put(key string, value []byte) keyval.BytesTxn {
  	tx.ops = append(tx.ops, clientv3.OpPut(key, string(value)))
  	return tx
  }
docs: // Put adds a new 'put' operation to a previously created transaction.
  // If the <key> does not exist in the data store, a new key-value item
  // will be added to the data store. If <key> exists in the data store,
  // the existing value will be overwritten with the <value> from this
  // operation.