Huh, you're right, good catch. It seems that the behavior of returning zero when all numbers are negative needs to be explicitly stated for the specification to be complete. I'll edit the description accordingly.

Can we really say that the specification is complete? It seems that the function that returns 0 only when the list is empty and the maximum otherwise (i.e. there is no special case about negative integers) obeys the same specification.

Yes, I would like to do more too. Unfortunately further analysis of the system is beyond me at the moment, as I am not that familiar with these stuff either, so I would have to learn much more anyway. So basically, expect more to come, but don't hold your breath.

I'm glad that you were having fun with it. I had so much fun too when I learned these.

I found this problem very interesting (I have never attented a logic class, so I always enjoy this kind of exercise where I can learn a bit more about the topic).
But I must admit I find the exercises too short w.r.t. the length of the "details" section. Would it be possible to reason more about the system (in this kata, or another)? That would allow to better understand the system, and have more fun playing with it.