Mail Goggles in Emacs

I wrote an elisp which implements Mail Goggles in Emacs.
Mail Goggles is an extension of gmail and it provides a confirmation before sending emails by some simple calculations in order to prevent sending inappropriate emails with little care.

Here is the elisp.
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(defvar mail-goggles-default-number-of-times 3
  "The number of times that you need before do actions.")

(defvar mail-goggles-calculate-function-list '(+ - * /)
  "The list of calculation functions for mail-goggles.")

(defun mail-goggles ()
  "Gmail-like mail goggles.
   You have to do calculation several times that is specified by
   `mail-goggles-default-number-of-times'.
   The calculation types are defined in `mail-goggles-calculate-function-list'."
  (block "calc"
    (let ((i 0))
      (while (< i mail-goggles-default-number-of-times)
        (if (not
             (let* ((calc-func (nth (random 4) mail-goggles-calculate-function-list))
                    (x (random 100))
                    (y 
                     (cond ((equal calc-func '/)
                            (let ((tmp (random 10)))
                              (while (= 0 tmp)
                                (setq tmp (random 10)))
                              tmp)
                            (random 10))
                           ((equal calc-func '*)
                            (random 10))
                           (t (random 100))))
                    (answer (funcall calc-func x y)))
               (= (read-number (format "Calculate %d%s%d: " x calc-func y))
                  answer)))
            (progn
              (message "You are tired, aren't you?")
              (return-from "calc" nil)))
        (setq i (1+ i)))
      t)))

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I'm using gnus as my mail client, so I added these few lines.
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(defadvice message-send-and-exit (around mail-goggles)
  "Do mail-goggles before sending a mail."
  (if (mail-goggles)
      ad-do-it))

(ad-activate 'message-send-and-exit)

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This advice enables you to do mail-goggles before sending an email.