(defun is-leap-year (year)
    (and (= (% year 4) 0)
	 (if (= (% year 100) 0) (= (% year 400) 0) t)))

(defun is-leap-year-1 (year)
    (and (= (% year 4) 0)
	 (implies (= (% year 100) 0) (= (% year 400) 0))))

(defun is-leap-year-2 (year)
    (and (= (% year 4) 0)
	 (iff (= (% year 100) 0) (= (% year 400) 0))))