(use-syntax (ice-9 syncase))
(define memoized-factorial
  (let ((memo '()))
    (lambda (n)
      (let ((memoized (assq n memo)))
        (if (not (equal? memoized #f))
            (cadr memoized)
            (let ((new-value
                   (if (< n 1)
                       1
                       (* n (memoized-factorial (- n 1))))))
              (set! memo (cons (list n new-value) memo))
              new-value))))))