(let fibo (fun (n) {
  (mut i 0)
  (mut a 0)
  (mut b 1)
  (while (< i n) {
    (let c (+ a b))
    (set a b)
    (set b c)
    (set i (+ 1 i)) })
  a }))

(assert (= 6765 (fibo 20)) "(fibo 20) == 6765")