updateable/insertable view having left joined tables

Hi,

I'm taking a first foray into writing rules, and am struggling with one
for a view that has a left joined table:

---<--------------------cut here---------------start------------------->---
CREATE TABLE shoes (
sh_id serial PRIMARY KEY,
sh_name text,
sh_avail integer
);

CREATE TABLE shoelaces (
sl_id serial PRIMARY KEY,
sh_id integer REFERENCES shoes,
sl_name text
);

INSERT INTO shoes (sh_name, sh_avail) VALUES ('sh1', 2);
INSERT INTO shoes (sh_name, sh_avail) VALUES ('sh2', 0);
INSERT INTO shoes (sh_name, sh_avail) VALUES ('sh3', 4);
INSERT INTO shoes (sh_name, sh_avail) VALUES ('sh4', 3);

INSERT INTO shoelaces (sh_id, sl_name) VALUES (1, 'sl1');
INSERT INTO shoelaces (sh_id, sl_name) VALUES (3, 'sl2');

SELECT * FROM shoes;
sh_id | sh_name | sh_avail
-------+---------+----------
1 | sh1 | 2
2 | sh2 | 0
3 | sh3 | 4
4 | sh4 | 3

SELECT * FROM shoelaces;
sl_id | sh_id | sl_name
-------+-------+---------
1 | 1 | sl1
2 | 3 | sl2

-- We create a view that could be used to easily insert data into
-- shoelaces table:

CREATE VIEW shoe AS
SELECT sh.sh_id, sh_name, sh_avail, sl_name
FROM shoes sh LEFT JOIN shoelaces sl USING (sh_id);

SELECT * FROM shoe;
sh_id | sh_name | sh_avail | sl_name
-------+---------+----------+---------
1 | sh1 | 2 | sl1
2 | sh2 | 0 |
3 | sh3 | 4 | sl2
4 | sh4 | 3 |
---<--------------------cut here---------------end--------------------->---

Say I want to update this view like:

UPDATE shoe SET sl_name = 'sl3' WHERE sh_id = 2;

The right (well, to me) thing to do would be to insert a row in
shoelaces like this:

INSERT INTO shoelaces (sh_id, sl_name) VALUES (2, 'sl3');

Of course, if the update involves a row that is already available from
shoelaces, it would be an update on at least one of the tables, rather
than an insert on shoelaces. I'm not sure how a rule to do this would
look like, so any pointers would be appreciated. Thanks.

--
Seb