Nested transactions

Dear postgresql hackers,

I'm trying to find out what happens if I use nested transactions in
postgresql from 7.2.x to 7.4.x. I visited and surfed across many webpages
and documents but I did not find anything (I did not look at the sources
yet). I know nested transactions did not work on that versions, but this
is exactly what I want to know. What happens if I put nested begins,
commits, etc? Suppose we have the following:

begin work
...
begin work
...
commit
...
commit

What happens with the second begin? What happens in the first and second
commits? And if I put a rollback inside the two nested transactions? What
happens? Thank you in advance,

[]'s

cassio.