@IBAction func equalitySignPressed(sender: UIButton) {

    if stillTyping {
        secondOperand = currentInput
    }

    dotIsPlaced = false

    switch operationSign {

    case "+":
        operateWithTwoOperands{$0 + $1}

    case "-":
        operateWithTwoOperands{$0 - $1}

    case "✕":
        operateWithTwoOperands{$0 * $1}

    case "÷":
        operateWithTwoOperands{$0 / $1}
    default: break

    }
}

«÷»  Operation.BinaryOperation (/, {$0 + «÷» + $1}, 1,
                             | { $1 == 0.0 ? «Division by zero» : nil })