1.19¶

(define (even? x) (= (remainder x 2) 0))
(define (fib n) 
    (fib-iter 1 0 0 1 n))
(define (fib-iter a b p q count)
    (cond ((= count 0) b)
          ((even? count)
           (fib-iter a
                     b
                     (+ (square p) (square q))
                     (+ (* 2 p q) (square q))
                     (/ count 2)))
          (else (fib-iter (+ (* b q) (* a q) (* a p))
                          (+ (* b p) (* a q))
                          p
                          q
                          (- count 1)))))

(fib 0) 
(fib 1) 
(fib 2) 
(fib 3) 
(fib 4) 
(fib 5) 
(fib 6) 
(fib 7) 
(fib 8) 
(fib 9) 
(fib 10)