(define (fib n #!optional cont)
  (define (fib n k)
    (if (< 0 n)
	(fib (quotient n 2)
	     (lambda (a b)
	       (let ((c (* a (- (* 2 b) a)))
		     (d (+ (* a a) (* b b))))
		 (if (= 0 (remainder n 2))
		     (k c d)
		     (k d (+ c d))))))
	(k 0 1)))
  (if (and (exact? n) (not (negative? n)) (integer? n))
      (fib n (if (default-object? cont) cons cont))
      (error ":not-exact-nonnegative-integer" n 'FIB)))