subtract method
return 0 then result is minus.
Implementation
Safety<IndentConstantPart> subtract(int amount) {
final log = Log(classLocation: runtimeType, functionLocation: 'subtract');
final newTimes = times - amount;
if (newTimes < 0) return Safety(IndentConstantPart(0), log);
return Safety(IndentConstantPart(newTimes), log);
}