How to implement transaction in plpgsql?

Dear all,

How to implement transaction in plpgsql?
I have try to use it but fail. And I don't know the reason.
Here is my function...

CREATE FUNCTION sp_templatetouser(INTEGER, INTEGER) RETURNS INTEGER AS '
 DECLARE
  puserid ALIAS FOR $1;
  pplateid ALIAS FOR $2;
  rec_affected INTEGER;
 BEGIN

  BEGIN WORK;
  DELETE FROM userrights WHERE userid = puserid;
  INSERT INTO userrights SELECT puserid, rightid, allow FROM platedetails
WHERE plateid = pplateid;

  GET DIAGNOSTICS rec_affected = ROW_COUNT;
  IF rec_affected = 3 THEN
   RETURN 1;
  ELSE
   ROLLBACK WORK;
   RETURN 0;
  END IF;
  COMMIT WORK;
 END;
' LANGUAGE 'plpgsql';

best regards,
Corn.