in product . fmap snd . filter (testBit n . fst) . zip [0 ..] . take (1 + floorLog (n + 1)) $ iterate (sqr' m) x

       in (foldr (mult' m . snd) 1 . filter (testBit n . fst) . zip [0 ..] . take (1 + floorLog (n + 1))) $ iterate (sqr' m) x