I'm NOT reading docs, or at least, I don't want to look at them each time I use a function.

Don't use the same word to mean two different things. If I ever have to use this function, my first guess would be that it's a AR filter, i.e. a lag polynomial. (`ar` versus `ar_coeffs` is very clear in my opinion.)

(However, pandas became very popular with 2 and 3 letter names, -- I still haven't figured it out.)