Let's look at two versions of this function:

(defun fib1 (n)
  (if (<  n 2)
      n
      (+ (fib1 (- n 2))
         (fib1 (- n 1)) ) ) )



(defun fib2 (n) (fib2b 0 1 n))

(defun fib2b (lo hi steps)
  (if (= steps 0)
      lo
      (fib2b hi (+ lo hi) (- steps 1)) ) )

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