(* Returns whether n is prime.
   Requires: n is a positive integer. *)
fun isPrime(n: int): bool =
  let fun noDivisorsAbove(m: int) =
    if n mod m = 0 then false
    else if m*m >= n then true
    else noDivisorsAbove(m+1)
  in
    noDivisorsAbove(2)
  end