yes, I use to work with functional languages that have not that issue, I guess that cloning the list could be enough for avoiding this issue BUT the problem isn't saying it's going to continue using that list or I've to keep the integrity of that list. Tests are passing and performing something that's not requested only "what if..." could be overengineering as well.

wow

this is bad, because it will sort numbers, and you might have a different "numbers" list after using this function