public NumberAdditionResult AddThree() => NumberAdditionResult.NoCarryResult(new Number9());
 public NumberAdditionResult AddOne() => NumberAdditionResult.NoCarryResult(new Number7());
 public NumberAdditionResult AddTwo() => NumberAdditionResult.NoCarryResult(new Number8());
 public NumberAdditionResult AddNumber(INumber number) => NumberAdditionResult.NoCarryResult(number);