#light

let fib n =
    let rec fib_aux (n, a, b) =
        match (n, a, b) with
          | (0, a, b) -> a
          | _ -> fib_aux (n - 1, a + b, a)
    fib_aux (n, 0, 1)
    
// BigInt version.

let big_fib n =
    let rec big_fib_aux (n, a, b) =
        match (n, a, b) with
          | (0, a, b) -> a
          | _ -> big_fib_aux (n - 1, a + b, a)
    big_fib_aux (n, 0I, 1I)