I'm personally against introducing functions (with the associated namespace pollution) for things that can easily be programmed at the user level just like you said. IMO, anyone for which factorial(n) is not obviously equivalent to gammafunc(n+1) shouldn't be writing functions in the first place. Of course, I'm fully open to be convinced of the contrary. You may say that, by the same logic, one could define floor() as f = (x>0) ? int(x) : -ceil(-x) but, like you said, floor() is a standard function that should have been there right from the start.