Are rules transaction safe?

Hi all,

I have a question regarding rules on views. Are the commands inside a ON
INSERT (or UPDATE) DO INSTEAD (implicit) transactions for postgres? I
tried to put BEGIN; and COMMIT; around the commands in a ON ... DO
INSTEAD ( ) block, but keep getting a syntax error.

The following example might explain what I'm trying to find out:

-------------------------------------------------------------
Two tables, "a" and "b". "b" is referencing "a" via "a_id"...

   CREATE TABLE a
   (
      id serial,
      foo varchar(255),
      CONSTRAINT aid PRIMARY KEY (id)
   );

   CREATE TABLE b
   (
      id serial,
      a_id int4 not null,
      foo varchar(255),
      CONSTRAINT bid PRIMARY KEY (id),
      CONSTRAINT bfk FOREIGN KEY (a_id)
         REFERENCES a (id) MATCH SIMPLE
         ON UPDATE CASCADE ON DELETE CASCADE
   );


...a view "ab" for a combination of the before mentioned tables...

   CREATE OR REPLACE VIEW ab AS
     SELECT a.id AS main_id, a.foo AS from_a, b.foo AS from_b
     FROM a, b
     WHERE a.id = b.a_id;

...and a rule "ab_insert"...

   CREATE OR REPLACE RULE ab_insert AS ON INSERT TO ab DO INSTEAD (
     INSERT INTO a (foo) VALUES (new.from_a);
     INSERT INTO b (foo,a_id) VALUES (new.from_b,currval('a_id_seq'));
   );
-------------------------------------------------------------

As you can see, the "ab_insert" rule inserts into "a" first and than
takes the current value of "a.id"'s sequence to set the reference "a_id"
in "b".

Can I assume that this will always work as expected or is it possible
that in a multi-user scenario two or more concurrent inserts on the view
will lead to undesirable results?

As mentioned in the beginning, putting BEGIN; and COMMIT; didn't work.
Is this kind of creating a relation between two or more tables and
relying on a sequence generally a good practice?

Regards,
Oliver