(defun !rules ()
  (clrhash *rules*)
  (<- (female janet))
  (<- (male tim))
  (<- (male donald))
  (<- (male gordon))
  (<- (parent tim    donald))
  (<- (parent janet  donald))
  (<- (parent gordon donald))
  (<- (father ?x ?y) (and (parent ?x ?y) (male ?y)))
  (<- (= ?x ?x))
  (<- (data (emps ((emp tim john) (emp susan jane)))))
  (<- (person ?x) (or (male ?x) (female ?x)))
  (<- (sibling ?x ?y) (and (person ?x)
			   (person ?y)
			   (not (= ?x ?y))
			   (parent ?x ?z)
			   (parent ?y ?z)))
  (<- (append nil ?xs ?xs)) 
  (<- (append (?x . ?xs) ?ys (?x . ?zs))
      (append ?xs ?ys ?zs))
)

(deftest !pl0 ()
  (!rules)
  (with-answer (data (emps ?base))
    (format t "(data ~a)~%" ?base)))

(deftest !pl1 ()
  (!rules)
  (with-answer (father ?x ?y)
    (format t "~a is the father of ~a. ~%" ?y ?x)))

(deftest !pl2()
  (!rules)
  (with-answer (sibling ?x ?y)
    (format t "~a is the sibling of ~a.~%" ?x ?y)))

(deftest !pl3 ()
  "shows how our prolog can't do lists"
  (!rules)
  (with-answer (append ?x (c d) (a b c d)) 
    (format t "Left: ~A~%" ?x))
  (with-answer (append (a b) ?x (a b c d))
    (format t "Right: ~A~%" ?x))
  (with-answer (append (a b) (c d) ?x)
    (format t "Whole: ~A~%" ?x))
  (with-answer (append ?x ?y (a b c)) 
    (format t "Left: ~A Right: ~A~%" ?x ?y)))