     
     @IBAction func appendDigit(sender: UIButton) {
         let digit = sender.currentTitle!
+
+        if digit  == "." && userHasEnteredADot == true {
+            return
+        }        
+        if digit  == "." && userHasEnteredADot == false {
+            userHasEnteredADot = true
+        }
         
         if userIsInTheMiddleOfTypingANumber {
             display.text = display.text! + digit

    
    @IBAction func enter() {
        userIsInTheMiddleOfTypingANumber = false
        operandStack.append(displayValue)
        println(operandStack)
    }

    @IBOutlet weak var display: UILabel!

    var userIsInTheMiddleOfTypingANumber = false
    var userHasEnteredADot = false
    //var operands = Array<Double>()
    var operandStack = [Double]()
    var displayValue: Double {
