Your solution is a good one, when I do this in Kata only according to their own ideas to deal with the problem. The realization way of thinking about their own, you do not use a string I don't want to see to achieve, but a more thorough understanding of the kata.
I don't throw the numbers around to make it easier to make additions because binary numbers can be added to the addition of my solution.
I am a Chinese, my English is not good, most of the need to use translation software to communicate, if there is inappropriate, please understand, thank you

Thank you very much for your advice. It is indeed a bug. I've fixed this problem, and I hope you like this Kata, although it seems to be a bit bad