anchor.diff:                      &key
                         (encoding :linear)
                         goal)
  -
     (labels ((location-predicate (object parent i j)
  -             (ecase encoding
  -               (:quadratic (list 'at object (itmp-encode-location parent i j)))
  -               (:linear `(= (position ,object) ,(itmp-encode-location parent i j)))))
  +             (let ((parent-frame (robray::scene-graph-lookup scene parent)))
  +               (cond
  +                 ((robray::scene-frame-geometry-isa parent-frame "surface")
  +                  (ecase encoding
  +                    (:quadratic (list 'at object (itmp-encode-location parent i j)))
  +                    (:linear `(= (position ,object) ,(itmp-encode-location parent i j)))))
  +                 ((robray::scene-frame-geometry-isa parent-frame "stackable")
  +                  (assert (eq encoding :linear))
  +                  `(= (position ,object) ,parent))
  +                 (t (error "Unkown type of parent ~A" parent)))))
              (occupied-predicate (parent i j)
                (list 'occupied (itmp-encode-location parent i j)))
              (frame-predicates (name parent i j)
  
positive.code: 
  
      (let* ((scene (scene-graph scene))
             (stackable-frames (scene-collect-type scene "stackable"))
             (locations-list
              (loop for frame in stackable-frames
                 for name = (robray::scene-frame-name frame)
  
negative.code:          (goal-scene (scene-graph goal-scene))
           (moveable-frames (scene-collect-type init-scene "moveable"))
           (moveable-objects (map 'list #'robray::scene-frame-name moveable-frames))
           (stackable-frames (scene-collect-type init-scene "stackable"))
           (stackable-objects (map 'list #'robray::scene-frame-name stackable-frames))
           ;;(stackable-objects (map 'list #'robray::scene-frame-name stackable-frames))
           (locations (scene-locations init-scene resolution
                                       :max-count max-locations
  