O(n) recursive Fibonacci

(define fib (lambda (n) (fib_h 1 0 n )))
(define fib_h (lambda (a b d)
    (cond
        ((= d 1)
            (+ a b)
        )
        (#t
            (fib_h b (+ a b) (- d 1))
        )
    )
))
(display (fib 500000))