@FXML
    public void btnHandler(ActionEvent event) {
        Button btn = (Button) event.getSource();
        new PulseTransition(btn).play();

        String id = btn.getId();
        String btnTxt = btn.getText();
        switch (id) {
            case "num":
                display.appendText(btnTxt);
                break;
            case "operand":
                operatorBtn(btnTxt);
                break;
            case "equalsBtn":
                equalsBtn();
                break;
            case "decimalBtn":
                decimalPtBtn();
                break;
            case "delBtn":
                display.removeLast();
                break;
            case "acBtn":
                display.clear();
                break;
            case "answerBtn":
                display.setText(calcEngine.lastAnswer);
                break;
            default:
                System.out.println("should not have done that");
        }
    }