final Button power = new ButtonOperator(controller, new OperatorPower());
g.setWidget(1, 4, power);
final Button invert = new ButtonOperator(controller,
new OperatorInvert());
g.setWidget(3, 1, invert);
// add special button for clear (handled directly by controller)
final Button clear = new Button(CalculatorConstants.CLEAR);
clear.addClickListener(new ClickListener() {