 
     // MARK: Window delegate
 
-    func windowWillClose(notification: NSNotification)
+    func windowWillClose(_ notification: Notification)
     {
         windowIsVisible = false
     }

        redisplayLetters()
    }

    override func showWindow(sender: AnyObject?)
    {
        super.showWindow(sender)
        windowIsVisible = true


    var applicationDelegate: AppDelegate
    {
        return NSApplication.shared.delegate as! AppDelegate
    }

    var windowIsVisible: Bool = true
