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

class ViewController: UIViewController {

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

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