;; creating rules
(defmacro rule-if (a op b then c)
  (cond ((equal op 'and)
	 `(lambda () (when (and (exist-in-facts? ',a) (exist-in-facts? ',b))
		       (add-fact ',c))))
	((equal op 'or)
	 `(lambda () (when (or (exist-in-facts? ',a) (exist-in-facts? ',b))
		       (add-fact ',c))))
	((equal op 'xor)
	 `(lambda () (when (or (and (exist-in-facts? ',a) (not (exist-in-facts? ',b)))
			       (and (not (exist-in-facts? ',a)) (exist-in-facts? ',b)))
		       (add-fact ',c))))))