Comparison shopping

Package: SHOPPING_PACKAGE. Registered by createEngine(); for a slimmer engine, register it explicitly (see choosing packages).

The shop-shelf question is which of two things is the better value. The maths for each side is already ordinary arithmetic: a stacked discount is £80 - 20% - 10%, and a per-unit price is a division, so £3 / 500g is the price per gram. vs adds the missing piece, putting two of them side by side.

£3 / 500g vs £4 / 750g // the second is cheaper, 11% less
£3 vs £4 // the first is cheaper, 25% less

Lower is cheaper, and the two sides have to be the same kind of thing: a price against a price, a per-gram rate against a per-gram rate. A price against a weight is reported as an error rather than a meaningless answer. versus is an alias, and two equal amounts read as the same.