anchor.diff:    (unless with-shockwave
       (setf (drawable explosion)
             nil)))
  -                                
  +
   ;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
   ;;;
   ;;; purely visual objects
  
positive.code:     (setf (slot-value (parent child) 'children)
            (remove child (children (parent child))))
      (setf (slot-value child 'parent) nil)))
    
  
  (defmethod acquaint ((parent composite) (children cons))
    (dolist (child children)
  
negative.code:   (when group
      (join-group unit group))
    (acquaint unit (weapons unit)))
  
  (defclass ground-unit (visible-circle unit)
    ((new-positions ;list of new positions created by bumping into other units. the centroid of these positions will be used as the new pos prior to move
      :initform ()
  