let unique xs =
    let rec f us = function (* TODO: make this not O(n^2) *)
        (x::xs) -> if (List.exists ((=) x) us) then f us xs else f (x::us) xs
      | [] -> us
    in List.rev (f [] xs)

let counter s =
    let n = ref (s-1) in
    fun () -> n := !n + 1; !n