@IBAction func operate(sender: UIButton) {
        let operation = sender.currentTitle

        if userIsInTheMiddleOfTypingANumber {
            enter()
        }

        switch operation { 
        case "×": performOperation(test)
        case "÷": performOperation { $0 / $1 }
        case "+": performOperation { $0 + $1 }
        case "−": performOperation { $0 - $1 }
        default: break
        }
    }