(defun month-length (mon y)
	   (case mon
	     ((jan mar may jul aug oct dec) 31)
	     ((apr jun sept nov) 30)
	     (feb (if (leap-year y)29 28))
	     (otherwise "unknown month")))

(defun leap-year (y)
  (and (zerop (mod y 4))
       (or (zerop (mod y 400))
           (not (zerop (mod y 100))))))

(deftest test-month-length ()
	   (check 
	     (equalp '31 (month-length 'jan '2000))
	     (equalp '29 (month-length 'feb '2004))
	     (equalp 't  (leap-year '2000))
	     (equalp 'nil (leap-year '1993))))