(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")