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@nsu.ru
Phone: +7-3832-397354