;;; mejja plays with let-values and dijkstras fib:
;;; www.cs.utexas.edu/users/EWD/ewd06xx/EWD654.PDF

(define (fib n)				;fantastic?
  (if (< 0 n)
      (let-values (((a b) (fib (quotient n 2))))
	(let ((c (* (- (* 2 b) a) a))
	      (d (+ (* b b) (* a a))))
	  (if (= 0 (remainder n 2))
	      (values c d)
	      (values d (+ c d)))))
      (values 0 1)))

(define (fib n)				;using let*-values
  ;; assumption: (div n d) => (values q r)
  (if (< 0 n)
      (let*-values (((q r) (div n 2)) ((a b) (fib q)))
	(let ((c (* a (- (* 2 b) a)))
	      (d (+ (* a a) (* b b))))
	  (if (= r 0)
	      (values c d)
	      (values d (+ c d)))))
      (values 0 1)))