Rules Question

I have a question on using INSERT rules.  What I am attempting to do is to
create a rule that checks to see if the item being inserted already exists.If it does exist, increase the stored
quantitybye the NEW.quantity,
 
otherwise, just handle the INSERT as normal.

Here is what I've tried

CREATE TABLE orders (cust_id            int,item_id            text,quantity        int
);

CREATE RULE combine_dupes AS ON INSERT TO orders   WHERE NEW.item_id IN       (SELECT item_id FROM orders       WHERE
cust_id= NEW.cust_id AND item_id = NEW.item_id)   DO        UPDATE orders SET quantity = NEW.quantity + quantity
WHEREcart_id = NEW.cart_id AND item_ID = NEW.item_id;
 


I've also tried it  as a DO INSTEAD and without the "AND item_id =
NEW.item_id" clause (which shouldn't be needed) but they don't work either.

So, what's the deal?  Am I making a syntax error, or do I need to use a
function or trigger in order to do what I'm trying?

BTW: Please respond or cc directly to me, I only subscribe to
pgsql-general, I just thought this was more SQL related.