nested transaction

Hello,

The new release includes savepoints, but I need to do a begin inside another
begin and a commit/rollback after another commit/rollback.

For example, I have a pA procedure that update some data, so this procedure
execute a begin, after manipulate data and ends with commit/rollback. pA
procedure can to be called directly by the client aplication. In other hand,
I have a pB procedure that also update some data and calls pA procedure, so
pB procedure execute a begin, after manipulate data, after call pA procedure
and ends with commit/rollback. pB procedure can also to be called directly
by the client aplication.

The problem is that pA procedure can't do a new transaction, so what can I
do?
Is possible to detect from pA that there is a transaction and so do a
savepoint instead of a begin?

Thanks in advance,

pere