thanks, that helped

is this case somehow special, so that it checks any kind of specific possible mistake in algorithm?

This kata made me cry. I started it two month ago, gave up, started again and again gave up, and on some iteration I've spend much more time for mathematical background than for all previous attempts... then at least the same ammount of time for debugging and optimization. And when everyting worked, tests passed... I've checked other solutions that were 4-5 times shorter than mine, I wanted to cry again, but I was drunk too much at that point.

Thanks for the kata :)