code: func (tx *Txn) Put(key string, value []byte) keyval.BytesTxn {
  	if tx.addPrefix != nil {
  		key = tx.addPrefix(key)
  	}
  	tx.ops = append(tx.ops, op{key, value, false})
  	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 the key exists in the data store,
  // the existing value will be overwritten with the value from this
  // operation.