insert into view

Hi

im trying to insert into the following view and i have problem handeling the
serial type

CREATE TABLE a (field11 serial PRIMARY KEY,field12 integer);
CREATE TABLE b (field11 integer REFERENCES field11, field22 integer PRIMARY
KEY)

CREATE VIEW some_view AS
    SELECT * FROM a,b left join b using (field11);

CREATE RULE insert_into_view AS ON INSERT TO some_view DO INSTEAD (
 INSERT INTO a (field12) VALUES (NEW.field11);
 INSERT INTO b (field11, field22) VALUES (NEW.field11, NEW.field22);
);

now.

how do i tell him the second insert in the rule to use the new value of the
serial from the first insert ?

Thanks in advance