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.

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.