triggers on prepare, commit, rollback... ?

Dear pgdev,

I've played with triggers a bit, and I have noticed that there seem to be
no way to add a trigger on events such as "prepare", "commit", "rollback",
"savepoint", if I'm not mistaken. Also, possible interesting events could
be "create", "alter" and so, but it may already be possible to catch these
by having a trigger on "pg_class" or the like.

It seems to me that such triggers would be useful to help implement a
"simple" (hmmm...) synchroneous replication system, possibly by extending
or modifying slony, or for advance logging.

Is there any special semantical issue for providing them in pg, or is it
just the matter of implementing the parser, bookkeeping, callbacks... but
with no other special "intrinsic" difficulty?

Thanks in advance,

--
Fabien.