cf property Null safety
final
        Implementation
static final NormalArgOperation cf = NormalArgOperation(
    arg: DigitArg(max: 5, calc: (model, arg) => model.setFlag(arg, false)),
    name: 'CF');
static final NormalArgOperation cf = NormalArgOperation(
    arg: DigitArg(max: 5, calc: (model, arg) => model.setFlag(arg, false)),
    name: 'CF');