code: func (tx *transaction) putKey(key, value []byte) error {
  	// Prevent the key from being deleted if it was previously scheduled
  	// to be deleted on transaction commit.
  	tx.pendingRemove.Delete(key)
  
  	// Add the key/value pair to the list to be written on transaction
  	// commit.
  	tx.pendingKeys.Put(key, value)
  	tx.notifyActiveIters()
  	return nil
  }
docs: // putKey adds the provided key to the list of keys to be updated in the
  // database when the transaction is committed.
  //
  // NOTE: This function must only be called on a writable transaction.  Since it
  // is an internal helper function, it does not check.