It puts the null terminator at the end of both strings.

Why? The return function exits the function immediately.

One can solve the kata without cheating and make a true Quine, I/O is not necessary so I dont think this hint would be a good thing.

Work be great for non constant a,b

A bit of a overkill, still beautiful nontheless.

You mean outside of this problem of adding goals? What would you suggest?

One downside with this is that you don't have a clue what you are adding together. (for instance, what is b?)

I think you need more secondary / random tests. My first solution passed but gets many wrong. There should be enough tests or a few tests with large numbers to encourage faster solutions.

Please see my other answer, this one is wrong when the most significant digit is two.

I admit this is a terible solution.

What "break" is not used here? What am i missing?