code: func (t *txn) Put(key string, value []byte) keyval.BytesTxn {
  	t.updates = append(t.updates, &update{
  		key:   []byte(key),
  		value: value,
  	})
  	return t
  }
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.