when statement
==============

    func addItemsToUse(ref Iterable<Item> items) {
        when itemsToUse != none
        #itemsToUse = .new()
        #itemsToUse.preallocate(items.length)
        itemsToUse = .new(items.length)
        for item in items {
            when item.type in [.Gadget, .Widget]
            when item.compatibleWith(something)
            itemsToUse.append(item)
        }
    }