;; compute the nth fibonacci number by memoization
(define mf
  (lambda (n)
    (let ((T (make-vector (+ n 1))))
      (define memoize
        (lambda (i)
          (if (not (vector-ref T i))
              (vector-set! T i
                             (if (< i 2)
                                 i
                                 (+ (memoize (- i 2)
                                    (memoize (- i 1)))))))
           (vector-ref T i)))
      (vector-fill! T #f)
      (memoize n))))