open Num

let fib n =
  let rec fib_aux f0 f1 count =
    match count with
      | 0 -> f0
      | 1 -> f1
      | _ -> fib_aux f1 (f1 +/ f0) (count - 1)
  in
    fib_aux (num_of_int 0) (num_of_int 1) n