are subtransactions not nestable?

Hello,

is it correct that a nesting of subtransactions is not possible with
postgres at the moment?
(I didn't find a TODO entry for this (even at 'exotic' features))

christof=> begin;
BEGIN
christof=> begin;
NOTICE: BeginTransactionBlock and not in default state
BEGIN
christof=> commit;
END
christof=> commit;
NOTICE: EndTransactionBlock and not inprogress/abort state
END

Regards
Christof