Yeah - the nature of this question makes me suspect an unclear understanding of the concept. No database would ever allow you to commit (ET) a transaction that you did a rollback (BT) to already. After a rollback (BT) you can re-execute the transactions again and then commit (ET) those.
Or the reverse, if you performed transactions and then did a commit (ET), you cannot then rollback (BT) to undo them. You would have to apply the transactions in reverse and then commit (ET) that to have the effect of a rollback (BT) after commit (ET).