(define (fib n)
  (cond ((= n 0) 0)
        ((= n 1) 1)
        (else (+ (fib (- n 1))
                 (fib (- n 2))))))

(define (fib2 n) (ifib 1 0 n))

(define (ifib next-fib cur-fib cnt)
  (if (= cnt 0)
      cur-fib
      (ifib (+ next-fib cur-fib)
            next-fib
            (- cnt 1))))