http://forums.bgdev.org/index.php?showtopic=44083

draw-cards.lisp

;; http://forums.bgdev.org/index.php?showtopic=44083

(defvar ace (/ 1 13))
(defvar heart0 (/ 13 51))
(defvar heart1 (/ 12 51))
(defvar heart (/ (+ heart1 (*  3 heart0)) 4))
(defvar probability (* ace heart))

;; Our deck of cards:
;;  0-12 -- spades (2 3 4 5 6 7 8 9 10 jack queen king ace)
;; 13-25 -- hearts
;; 26-38 -- diamonds
;; 39-51 -- clubs
(defun ace? (n)
  (member n '(12 25 38 51)))

(defun heart? (n)
  (and (<= 13 n) (<= n 25)))

(setq *random-state* (make-random-state t))
(defun draw-card (&optional selected)
  (let ((r (random 52)))
    (if (eql r selected)
        (draw-card selected)
      r)))

(defun draw-and-check ()
  (let* ((first (draw-card))
         (second (draw-card first)))
    (and (ace? first) (heart? second))))

(defun run (runs)
  (let ((wins 0))
    (loop for i from 1 to runs do
          (setq wins (+ wins (if (draw-and-check) 1 0))))
    (/ wins runs)))

(progn
  (princ "Expected: ")
  (princ (float (* 100 probability)))
  (fresh-line)

  (princ "Actual (10M runs): ")
  (princ (float (* 100 (run 10000000))))
  (fresh-line))