struct InventoryList< T > {
   var items: [T]

   mutating func add(item: T) {
       items.append(item)
   }

   mutating func remove() -> T {
       return items.removeLast()
   }

   func isCapacityLow() -> Bool {
       return items.count < 3
   }

   func numberOfItems() -> Int {
       return items.count
   }
}