serial nextval() question

I have the following table def:

create table TMP_LI (

cart_id integer references TMP_CART(id),
li_id integer not null,
shop_id integer references CHARISMA_SHOPS(id),
prod_id char(12) references PRODUCTS(id),
quantity int2 not null,
price integer not null,

primary key (cart_id, li_id)
);

I'd like to have li_id be an serial but not for the whole table, only as
li_id relates to cart_id ... i.e. I'd like li_id to increment relative
to cart_id so that I can generate sequences of the kind:

cart_id li_id
1 1
1 2
1 3
2 1
2
2

How could I do this?

Thanks!

Jc