(defun fibonacci (n)
  (let ( (vec) (i) (j) (k) )
    (if (< n 2) n
      (progn
	(setq vec (make-vector (+ n 1) 0) i 0 j 1 k 2)
	(setf (aref vec 1) 1)
	(while (<= k n)
	  (setf (aref vec k) (+ (elt vec i) (elt vec j) ))
	  (setq i (+ 1 i) j (+ 1 j) k (+ 1 k) ))
	(elt vec n) ))))