recursed transactions ?

Hello,
Does anybody know, if it possible to make recursed transactions like that:
BEGIN
INSERT ...
UPDATE ...
BEGIN
SELECT ...
UPDATE ....
END
END

psql - tells me:
ser=>begin;
BEGIN
ser=> begin;
NOTICE: BeginTransactionBlock and not in default state
ser=> end;
END
ser=> end;
NOTICE: EndTransactionBlock and not inprogress/abort state
END

Thank you !
---------------------------
Sergei Chernev
Internet: ser(at)nsu(dot)ru
Phone: +7-3832-397354