anchor.diff:          | ShopSell
           | ShopPageUp
           | ShopPageDown
  -        | ShopSelect of Lens<int * ItemType, World>
  +        | ShopSelect of Lens<int * (ItemType * int Option), World>
           | ShopConfirmAccept
           | ShopConfirmDecline
           | ShopLeave
  
positive.code: 
              | MenuItemSelect selectionLens ->
                  let selection = Lens.get selectionLens world
                  let field = Field.updateMenu (fun menu -> { menu with MenuUseOpt = MenuUse.tryMakeFromSelection selection }) field
                  just field
  
  
negative.code: 
              | ShopSelect selectionLens ->
                  let selection = Lens.get selectionLens world
                  let selection = (fst selection, fst (snd selection))
                  let field =
                      Field.updateShopOpt (Option.map (fun shop ->
                          let buying = match shop.ShopState with ShopBuying -> true | ShopSelling -> false
  