(def fib-iter (fn (n a b)
  (if (= n 0)
      a
      (fib-iter (- n 1) b (+ a b)))))
(def fib (fn (n) (fib-iter n 0 1)))

(write "(fib 50) = ")
(println (fib 50))

(def sum
  (fn (n acc)
    (if (= n 0)
      acc
      (sum (- n 1) (+ n acc)))))

(write "(sum 1000000) = ")
(println (sum 1000000 0))