I loved the kata but 4 kyu is a bit too much. I'd rank it 5 kyu max.

This kata is SOO GOOD!

I've spent a whole day figuring out how to optimize the code, since it timed-out at random tests. It made me go through the rabbit hole of finding out how the summation of digits works and how to MAX the result and minimize the number of iterations.

Honestly, this mixture of frustration and EUREKA! is what I'm looking for.

Riiiiight... I really had to think on this one. Coming from a Python community I 100% thought it meant "define a variable of type 'str', i.e. 'type string' in Python. All good now.

Thank you for your help :)

Ah, got it! I misread the second group and thought there are OO followed by 1 or more X (and vice versa with XX in the beginning).

Considering the example shown: 'XOOOXXO'

Shouldn't it count 4 Letter with Tails/Heads as well?

(XOOO)XXO, XO(OOXX)O

Which would increase the number of combinations from 26 to 28?