#old-style
(operator
  PICK-UP
  (params (<rob> ROBOT) (<ob1> OBJECT))
  (preconds 
	(clear <ob1>) (on-table <ob1>) (arm-empty <rob>))
  (effects 
	(holding <rob> <ob1>)))

(operator
  PUT-DOWN
  (params (<rob> ROBOT) (<ob> OBJECT))
  (preconds
	(holding <rob> <ob>))
  (effects 
	 (clear <ob>) (arm-empty <rob>) (on-table <ob>)))

(operator
 STACK
 (params (<rob> ROBOT) (<ob> OBJECT) (<underob> OBJECT))
 (preconds 
	(clear <underob>) (holding <rob> <ob>))
 (effects 
       (arm-empty <rob>) (clear <ob>) (on <ob> <underob>)))

(operator
 UNSTACK
 (params (<rob> ROBOT) (<ob> OBJECT) (<underob> OBJECT))
 (preconds
	(on <ob> <underob>) (clear <ob>) (arm-empty <rob>))
 (effects 
        (holding <rob> <ob>) (clear <underob>)))