#26 Group Discount operator is fragile

[Iain Sinclair]

operators.GroupDiscount is used to apply a discount
when a certain number of items is purchased.

Unfortunately the logic used is too simplistic and it only
works in the most basic situation where you enter the
correct quantity first off.

For example, if the discount applies to buying 3 of an
item, you enter the item with quanity 1, then yu enter
the item with quantity 3 = no discount.

If you enter the item with quantity 1, 3 times = no
discount.

It should be fairly obvious what is required to be done
here but I doubt that I'll get around to it... anybody?