 class ViewController: UIViewController {
 
     // implicitly (automatically) unwraps display
-    @IBOutlet weak var display: UILabel!
+    @IBOutlet private weak var display: UILabel!
     
-    var userIsInTheMiddleOfTyping = false
+    private var userIsInTheMiddleOfTyping = false
     
-    @IBAction func touchDigit(sender: UIButton) {
+    @IBAction private func touchDigit(sender: UIButton) {
         let digit = sender.currentTitle!
         if userIsInTheMiddleOfTyping {
             let textCurrentlyInDisplay = display.text!

        }
    }
    
    @IBAction func performOperation(sender: UIButton) {
        userIsInTheMiddleOfTyping = false
        if let mathematicalSymbol = sender.currentTitle {
            if mathematicalSymbol == "π" {

    }
    
    // computed propery
    private var displayValue: Double {
        get {
            return Double(display.text!)!
        }
