[ic] Cart update is freaky

[cHao]

A client of ours has a product people will order in multiple sizes at once.
For confidence's sake, let's assume they're T-shirts.  Someone browsing the
site might want to buy 10 large, 10 medium, and 10 small shirts.
Interchange adds them to the cart, and all is well so far.

(For reference, the size is a simple option with a price differential.  Each
size of shirt is a different price, but all share the same SKU.)

The problem comes when the buyer changes their mind and decides they only
want 8 medium shirts.  The way the current cart component and checkout form
seem to work, they don't know how to remove or update any but the first item
in the cart.  I assume this is because the products in the cart all have the
same SKU, but i'm not sure.

I'm not too familiar with how Interchange processes forms, and quite frankly
the documentation is no help at all.  How do i unconfuse the form processor?

/daniel