Yet, this mutation is quite harmless for it adds an additional element and it doesn't actually modify any existing data. In that specific case, I would assume that adding data is actualy the requirement rather than not.

Absolutely true, I was searching for linear extrapolation furmula, before I took a look here.

I can't see how it uses more sum() calls that the regular loop solutions provided.

Indeed, it is much simpler than it seemed at first, I managed to solve it, thanks for the reply.

I have the same problem as @eshis with python, everything seems to work excluding last random test.