Every chain's price for a cart of picked products

Prices a shopper's own cart in one request, keyed p:<gtin>. Each line carries an equivalence: class means every chain is quoted its own cheapest product in the same class and pack band, which is what makes the totals comparable; exact means only the chains publishing that barcode are priced, because for that product the taxonomy cannot name a substitute worth standing behind. Unknown barcodes are dropped rather than rejected.