g.setWidget(2, 3, subtract);
final Button add = new ButtonOperator(controller, new OperatorAdd());
g.setWidget(3, 3, add);
final Button sqrt = new ButtonOperator(controller, new OperatorSqrt());
g.setWidget(0, 4, sqrt);
final Button power = new ButtonOperator(controller, new OperatorPower());
g.setWidget(1, 4, power);