let rec take n = function
  | x :: xs when n > 0 -> x :: take (n - 1) xs
  | _ -> []
let rec drop n = function
  | _ :: xs when n > 0 -> drop (n - 1) xs
  | xs -> xs
let rec partition n step = function
  | [] -> []
  | xs -> take n xs :: partition n step (drop step xs)