This feature was suggested by Gunther Kreuzberger at http://moodle.org/mod/forum/discuss.php?d=128473. As a new feature, it should go the DEV backlog (2.1), I know. But I just spent 2 days fixing some bug in this part of code and it takes some time to dive into this procedure. Therefore I implemented it now when I see deeply into this code. Otherwise, I would have to spend significant amount of time next time again. I hope that reviewers would understand that.