Insert rule and default values for PK

Hi,

I've a prolem inserting records in a view using different ways.
I want to insert either way, with PK given and without PK which should then
be taken by its DEFAULT stanza:
insert into a_and_b(a) values (537)    # id not given, self assigned
insert into a_and_b(x) values (true)    # id not given, self assigned
insert into a_and_b(x,id) values (true,55)   # id given, take that

For this example I created two tables. Note the Serial at tbla:

CREATE TABLE tbla
(id int4 NOT NULL,a int4,b varchar(12), CONSTRAINT tbla_pk PRIMARY KEY (id)
) 

CREATE TABLE tblb
(id int4 NOT NULL,x bool, y timestamp,CONSTRAINT tblb_pk PRIMARY KEY (id),CONSTRAINT tblb_fk FOREIGN KEY (id)
REFERENCEStbla (id) ON UPDATE CASCADE
 
ON DELETE CASCADE
) 

My view is defined as to concatinate this 1:1 relation:
CREATE OR REPLACE VIEW a_and_b AS SELECT tbla.id, tbla.a, tbla.b, tblb.x, tblb.y  FROM tbla NATURAL LEFT OUTER JOIN
tblb;




In the first run I tried this insert rule. But it turned out that I can't
insert rows, if I omit id
CREATE OR REPLACE RULE a_b_insert AS   ON INSERT TO a_and_b DO INSTEAD (    INSERT INTO tbla (id, a, b) VALUES (new.id,
new.a,new.b);   INSERT INTO tblb (id, x, y) VALUES (new.id, new.x, new.y););
 


So I had a try with that rule which already does what was intended. I can
now choose of wheter I want to supply id of if not.
However I get disturbed by the 'nextval' and 'curval'-functions. This is
redundant with the tables definition. I don't like that.
The other way round, leave all id out, helps for tbla (since a default is
defined) but leaves tblb alone:

CREATE OR REPLACE RULE a_b_insert AS   ON INSERT TO a_and_b DO INSTEAD ( INSERT INTO tbla (id, a, b)  VALUES
(coalesce(new.id,nextval('public.tbla_id_seq'::text)),new.a,
 
new.b);INSERT INTO tblb (id, x, y)  VALUES (coalesce(new.id,currval('public.tbla_id_seq'::text)), new.x,
new.y);
);



What would you do? What is the postgres way of solving this problem.