; Recursive Fibonacci numbers
(define (fib n)
  (cond ((= n 0) 0)
        ((= n 1) 1)
        (else (+ (fib (- n 1))
                 (fib (- n 2)))) ) )

; Tail-recursive Fibonacci using helper function
(define (fibon n)
   (fib-h n 0 0 1))

(define (fib-h goal current fib-current fib-next)
   (if (= goal current)
       fib-current
       (fib-h goal (1+ current) fib-next
              (+ fib-current fib-next))))

; Iterative Fibonacci
(define (fibonacci n)
  (let ((lastfib 0) (thisfib 1) (nextfib 0))
    (dotimes (i n lastfib)
      (set! nextfib (+ lastfib thisfib))
      (set! lastfib thisfib)
      (set! thisfib nextfib) ) ))

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