Here is a solution to the classic Monkey and Bananas problem implemented in Lisa:

  (lisa:consider-taxonomy)
  (defclass mab-fundamental () ())
  (defclass monkey (mab-fundamental)
    ((location :initarg :location
	       :initform 'green-couch)
     (on-top-of :initarg :on-top-of
		:initform 'floor)
     (satisfied :initarg :satisfied
		:initform nil)
     (holding :initarg :holding
	      :initform 'nothing)))
  (defclass thing (mab-fundamental)
    ((name :initarg :name)
     (location :initarg :location)
     (on-top-of :initarg :on-top-of
		:initform 'floor)
     (weight :initarg :weight
	     :initform 'light)))
  (defclass chest (mab-fundamental)
    ((name :initarg :name)
     (contents :initarg :contents)
     (unlocked-by :initarg :unlocked-by)))
  (defclass goal-is-to (mab-fundamental)
    ((action :initarg :action)
     (argument-1 :initarg :argument-1)
     (argument-2 :initarg :argument-2
		 :initform nil)))
  ;;;(watch :activations)
  ;;;(watch :facts)
  ;;;(watch :rules)
  ;;; Chest-unlocking rules...
  (defrule hold-chest-to-put-on-floor ()
    (goal-is-to (action unlock) (argument-1 ?chest))
    (thing (name ?chest) (on-top-of (not floor)) (weight light))
    (monkey (holding (not ?chest)))
    (not (goal-is-to (action hold) (argument-1 ?chest)))
    =>
    (assert ((make-instance 'goal-is-to :action 'hold :argument-1 ?chest))))
  (defrule put-chest-on-floor ()
    (goal-is-to (action unlock) (argument-1 ?chest))
    (?monkey (monkey (location ?place) (on-top-of ?on) (holding ?chest)))
    (?thing (thing (name ?chest)))
    =>
    (format t "Monkey throws the ~A off the ~A onto the floor.~%" ?chest ?on)
    (modify ?monkey (holding blank))
    (modify ?thing (location ?place) (on-top-of floor)))
  (defrule get-key-to-unlock ()
    (goal-is-to (action unlock) (argument-1 ?obj))
    (thing (name ?obj) (on-top-of floor))
    (chest (name ?obj) (unlocked-by ?key))
    (monkey (holding (not ?key)))
    (not (goal-is-to (action hold) (argument-1 ?key)))
    =>
    (assert ((make-instance 'goal-is-to :action 'hold :argument-1 ?key))))
  (defrule move-to-chest-with-key ()
    (goal-is-to (action unlock) (argument-1 ?chest))
    (thing (name ?chest) (location ?cplace) (on-top-of floor))
    (monkey (location (not ?cplace)) (holding ?key))
    (chest (name ?chest) (unlocked-by ?key))
    (not (goal-is-to (action walk-to) (argument-1 ?cplace)))
    =>
    (assert ((make-instance 'goal-is-to :action 'walk-to :argument-1 ?cplace))))
  (defrule unlock-chest-with-key ()
    (?goal (goal-is-to (action unlock) (argument-1 ?name)))
    (?chest (chest (name ?name) (contents ?contents) (unlocked-by ?key)))
    (thing (name ?name) (location ?place) (on-top-of ?on))
    (monkey (location ?place) (on-top-of ?on) (holding ?key))
    =>
    (format t "Monkey opens the ~A with the ~A revealing the ~A.~%"
	    ?name ?key ?contents)
    (modify ?chest (contents nothing))
    (assert ((make-instance 'thing :name ?contents :location ?place
			    :weight 'light :on-top-of ?name)))
    (retract ?goal))
  ;;; Hold-object rules...
  (defrule unlock-chest-to-hold-object ()
    (goal-is-to (action hold) (argument-1 ?obj))
    (chest (name ?chest) (contents ?obj))
    (not (goal-is-to (action unlock) (argument-1 ?chest)))
    =>
    (assert ((make-instance 'goal-is-to :action 'unlock :argument-1 ?chest))))
  (defrule use-ladder-to-hold ()
    (goal-is-to (action hold) (argument-1 ?obj))
    (thing (name ?obj) (location ?place) (on-top-of ceiling) (weight light))
    (not (thing (name ladder) (location ?place)))
    (not (goal-is-to (action move) (argument-1 ladder) (argument-2 ?place)))
    =>
    (assert ((make-instance 'goal-is-to :action 'move
			    :argument-1 'ladder
			    :argument-2 ?place))))
  (defrule climb-ladder-to-hold ()
    (goal-is-to (action hold) (argument-1 ?obj))
    (thing (name ?obj) (location ?place) (on-top-of ceiling) (weight light))
    (thing (name ladder) (location ?place) (on-top-of floor))
    (monkey (on-top-of (not ladder)))
    (not (goal-is-to (action on) (argument-1 ladder)))
    =>
    (assert ((make-instance 'goal-is-to :action 'on :argument-1 'ladder))))
  (defrule grab-object-from-ladder ()
    (?goal (goal-is-to (action hold) (argument-1 ?name)))
    (?thing (thing (name ?name) (location ?place) 
		   (on-top-of ceiling) (weight light)))
    (thing (name ladder) (location ?place))
    (?monkey (monkey (location ?place) (on-top-of ladder) (holding blank)))
    =>
    (format t "Monkey grabs the ~A.~%" ?name)
    (modify ?thing (location held) (on-top-of held))
    (modify ?monkey (holding ?name))
    (retract ?goal))
  (defrule climb-to-hold ()
    (goal-is-to (action hold) (argument-1 ?obj))
    (thing (name ?obj) (location ?place (not ceiling))
	   (on-top-of ?on) (weight light))
    (monkey (location ?place) (on-top-of (not ?on)))
    (not (goal-is-to (action on) (argument-1 ?on)))
    =>
    (assert ((make-instance 'goal-is-to :action 'on :argument-1 ?on))))
  (defrule walk-to-hold ()
    (goal-is-to (action hold) (argument-1 ?obj))
    (thing (name ?obj) (location ?place) (on-top-of (not ceiling))
	   (weight light))
    (monkey (location (not ?place)))
    (not (goal-is-to (action walk-to) (argument-1 ?place)))
    =>
    (assert ((make-instance 'goal-is-to :action 'walk-to :argument-1 ?place))))
  (defrule drop-to-hold ()
    (goal-is-to (action hold) (argument-1 ?obj))
    (thing (name ?obj) (location ?place) (on-top-of ?on) (weight light))
    (monkey (location ?place) (on-top-of ?on) (holding (not blank)))
    (not (goal-is-to (action hold) (argument-1 blank)))
    =>
    (assert ((make-instance 'goal-is-to :action 'hold :argument-1 'blank))))
  (defrule grab-object ()
    (?goal (goal-is-to (action hold) (argument-1 ?name)))
    (?thing (thing (name ?name) (location ?place) 
		   (on-top-of ?on) (weight light)))
    (?monkey (monkey (location ?place) (on-top-of ?on) (holding blank)))
    =>
    (format t "Monkey grabs the ~A.~%" ?name)
    (modify ?thing (location held) (on-top-of held))
    (modify ?monkey (holding ?name))
    (retract ?goal))
  (defrule drop-object ()
    (?goal (goal-is-to (action hold) (argument-1 blank)))
    (?monkey (monkey (location ?place) (on-top-of ?on) 
		     (holding ?name (not blank))))
    (?thing (thing (name ?name)))
    =>
    (format t "Monkey drops the ~A.~%" ?name)
    (modify ?monkey (holding blank))
    (modify ?thing (location ?place) (on-top-of ?on))
    (retract ?goal))
  ;;; Move-object rules...
  (defrule unlock-chest-to-move-object ()
    (goal-is-to (action move) (argument-1 ?obj))
    (chest (name ?chest) (contents ?obj))
    (not (goal-is-to (action unlock) (argument-1 ?chest)))
    =>
    (assert ((make-instance 'goal-is-to :action 'unlock :argument-1 ?chest))))
  (defrule hold-object-to-move ()
    (goal-is-to (action move) (argument-1 ?obj) (argument-2 ?place))
    (thing (name ?obj) (location (not ?place)) (weight light))
    (monkey (holding (not ?obj)))
    (not (goal-is-to (action hold) (argument-1  ?obj)))
    =>
    (assert ((make-instance 'goal-is-to :action 'hold :argument-1 ?obj))))
  (defrule move-object-to-place ()
    (goal-is-to (action move) (argument-1 ?obj) (argument-2 ?place))
    (monkey (location (not ?place)) (holding ?obj))
    (not (goal-is-to (action walk-to) (argument-1 ?place)))
    =>
    (assert ((make-instance 'goal-is-to :action 'walk-to :argument-1 ?place))))
  (defrule drop-object-once-moved ()
    (?goal (goal-is-to (action move) (argument-1 ?name) (argument-2 ?place)))
    (?monkey (monkey (location ?place) (holding ?obj)))
    (?thing (thing (name ?name) (weight light)))
    =>
    (format t "Monkey drops the ~A.~%" ?name)
    (modify ?monkey (holding blank))
    (modify ?thing (location ?place) (on-top-of floor))
    (retract ?goal))
  (defrule already-moved-object ()
    (?goal (goal-is-to (action move) (argument-1 ?obj) (argument-2 ?place)))
    (thing (name ?obj) (location ?place))
    =>
    (retract ?goal))
  ;;; Walk-to-place rules...
  (defrule already-at-place ()
    (?goal (goal-is-to (action walk-to) (argument-1 ?place)))
    (monkey (location ?place))
    =>
    (retract ?goal))
  (defrule get-on-floor-to-walk ()
    (goal-is-to (action walk-to) (argument-1 ?place))
    (monkey (location (not ?place)) (on-top-of (not floor)))
    (not (goal-is-to (action on) (argument-1 floor)))
    =>
    (assert ((make-instance 'goal-is-to :action 'on :argument-1 'floor))))
  (defrule walk-holding-nothing ()
    (?goal (goal-is-to (action walk-to) (argument-1 ?place)))
    (?monkey (monkey (location (not ?place)) (on-top-of floor) (holding blank)))
    =>
    (format t "Monkey walks to ~A.~%" ?place)
    (modify ?monkey (location ?place))
    (retract ?goal))
  (defrule walk-holding-object ()
    (?goal (goal-is-to (action walk-to) (argument-1 ?place)))
    (?monkey (monkey (location (not ?place)) (on-top-of floor) (holding ?obj)))
    (thing (name ?obj))
    =>
    (format t "Monkey walks to ~A holding the ~A.~%" ?place ?obj)
    (modify ?monkey (location ?place))
    (retract ?goal))
  ;;; Get-on-object rules...
  (defrule jump-onto-floor ()
    (?goal (goal-is-to (action on) (argument-1 floor)))
    (?monkey (monkey (on-top-of ?on (not floor))))
    =>
    (format t "Monkey jumps off the ~A onto the floor.~%" ?on)
    (modify ?monkey (on-top-of floor))
    (retract ?goal))
  (defrule walk-to-place-to-climb ()
    (goal-is-to (action on) (argument-1 ?obj))
    (thing (name ?obj) (location ?place))
    (monkey (location (not ?place)))
    (not (goal-is-to (action walk-to) (argument-1 ?place)))
    =>
    (assert ((make-instance 'goal-is-to :action 'walk-to :argument-1 ?place))))
  (defrule drop-to-climb ()
    (goal-is-to (action on) (argument-1 ?obj))
    (thing (name ?obj) (location ?place))
    (monkey (location ?place) (holding (not blank)))
    (not (goal-is-to (action hold) (argument-1 blank)))
    =>
    (assert ((make-instance 'goal-is-to :action 'hold :argument-1 'blank))))
  (defrule climb-indirectly ()
    (goal-is-to (action on) (argument-1 ?obj))
    (thing (name ?obj) (location ?place) (on-top-of ?on))
    (monkey (location ?place) (on-top-of ?top
					 (and (not (eq ?top ?on))
					      (not (eq ?top ?obj))))
	    (holding blank))
    (not (goal-is-to (action on) (argument-1 ?on)))
    =>
    (assert ((make-instance 'goal-is-to :action 'on :argument-1 ?on))))
  (defrule climb-directly ()
    (?goal (goal-is-to (action on) (argument-1 ?obj)))
    (thing (name ?obj) (location ?place) (on-top-of ?on))
    (?monkey (monkey (location ?place) (on-top-of ?on) (holding blank)))
    =>
    (format t "Monkey climbs onto the ~A.~%" ?obj)
    (modify ?monkey (on-top-of ?obj))
    (retract ?goal))
  (defrule already-on-object ()
    (?goal (goal-is-to (action on) (argument-1 ?obj)))
    (monkey (on-top-of ?obj))
    =>
    (retract ?goal))
  ;;; Eat-object rules...
  (defrule hold-to-eat ()
    (goal-is-to (action eat) (argument-1 ?obj))
    (monkey (holding (not ?obj)))
    (not (goal-is-to (action hold) (argument-1 ?obj)))
    =>
    (assert ((make-instance 'goal-is-to :action 'hold :argument-1 ?obj))))
  (defrule satisfy-hunger ()
    (?goal (goal-is-to (action eat) (argument-1 ?name)))
    (?monkey (monkey (holding ?name)))
    (?thing (thing (name ?name)))
    =>
    (format t "Monkey eats the ~A.~%" ?name)
    (modify ?monkey (holding blank) (satisfied t))
    (retract ?goal)
    (retract ?thing))
  (defrule monkey-is-satisfied ()
    (monkey (satisfied t) (:object ?monkey))
    =>
    (format t "Monkey is satisfied: ~S~%" ?monkey))
  ;;; Retract every object whose ancestor is an instance of MAB-FUNDAMENTAL...
  (defrule cleanup (:salience -100)
    (?fact (mab-fundamental))
    =>
    (retract ?fact))
  ;;; startup rule...
  (defrule startup ()
    =>
    (assert
     ((make-instance 'monkey :location 't5-7
		    :on-top-of 'green-couch
		    :location 'green-couch
		    :holding 'blank)))
    (assert
     ((make-instance 'thing :name 'green-couch
		    :location 't5-7
		    :weight 'heavy
		    :on-top-of 'floor)))
    (assert
     ((make-instance 'thing :name 'red-couch
		    :location 't2-2
		    :weight 'heavy
		    :on-top-of 'floor)))
    (assert
     ((make-instance 'thing :name 'big-pillow
		    :location 't2-2
		    :weight 'light
		    :on-top-of 'red-couch)))
    (assert
     ((make-instance 'thing :name 'red-chest
		    :location 't2-2
		    :weight 'light
		    :on-top-of 'big-pillow)))
    (assert
     ((make-instance 'chest :name 'red-chest
		    :contents 'ladder
		    :unlocked-by 'red-key)))
    (assert
     ((make-instance 'thing :name 'blue-chest
		    :location 't7-7
		    :weight 'light
		    :on-top-of 'ceiling)))
    (assert
     ((make-instance 'thing :name 'grapes
		    :location 't7-8
		    :weight 'light
		    :on-top-of 'ceiling)))
    (assert
     ((make-instance 'chest :name 'blue-chest
		    :contents 'bananas
		    :unlocked-by 'blue-key)))
    (assert
     ((make-instance 'thing :name 'blue-couch
		    :location 't8-8
		    :weight 'heavy
		    :on-top-of 'floor)))
    (assert
     ((make-instance 'thing :name 'green-chest
		    :location 't8-8
		    :weight 'light
		    :on-top-of 'ceiling)))
    (assert
     ((make-instance 'chest :name 'green-chest
		    :contents 'blue-key
		    :unlocked-by 'red-key)))
    (assert
     ((make-instance 'thing :name 'red-key
		    :location 't1-3
		    :weight 'light
		    :on-top-of 'floor)))
    (assert
     ((make-instance 'goal-is-to :action 'eat
		    :argument-1 'bananas))))