anchor.diff:  
   ;;;; Proxy for alien instances
   
  -#+clisp
  -(defvar *foreign-instance-locations* (make-hash-table :weak :key))
  +#?(or (sbcl>= 0 9 17) (featurep :clisp))
  +(defvar *foreign-instance-locations* 
  +  (make-hash-table #+clisp :weak #+sbcl :weakness :key))
   
   ;; TODO: add a ref-counted-proxy subclass
   (eval-when (:compile-toplevel :load-toplevel :execute)
     (defclass proxy (virtual-slots-object)
  -    (#-clisp(location :special t :type pointer))
  +    (#?-(or (sbcl>= 0 9 17) (featurep :clisp))(%location :special t :type pointer))
       (:metaclass virtual-slots-class)))
   
   (defgeneric instance-finalizer (instance))
  
positive.code: (defgeneric invalidate-instance (instance &optional finalize-p))
  (defgeneric allocate-foreign (object &key &allow-other-keys))
  
  (defun foreign-location (instance)
    #-clisp(slot-value instance 'location)
    #+clisp(gethash instance *foreign-instance-locations*))
  
  (defun (setf foreign-location) (location instance)
    #-clisp(setf (slot-value instance 'location) location)
    #+clisp(setf (gethash instance *foreign-instance-locations*) location))
  
  (defun proxy-valid-p (instance)
    #-clisp(slot-boundp instance 'location)
    #+clisp(and (gethash instance *foreign-instance-locations*) t))
  
  (defmethod reference-function ((name symbol))
    (reference-function (find-class name)))
  
negative.code:     #'(lambda ()
  	(funcall unref location))))
  
  ;; FINALIZE-P should always be the same as the keyword argument
  ;; :FINALZIE given to MAKE-PROXY-INSTANCE or non NIL if the proxy was
  ;; created with MAKE-INSTANCE
  (defmethod invalidate-instance ((instance proxy) &optional finalize-p)
    (remove-cached-instance (foreign-location instance))
    #+(or sbcl cmu)
    (progn
      (when finalize-p
        (funcall (instance-finalizer instance)))
      #?-(sbcl>= 0 9 17)(slot-makunbound instance '%location)
      #?(sbcl>= 0 9 17)(remhash instance *foreign-instance-locations*)
      (cancel-finalization instance))
    ;; We can't cache invalidated instances in CLISP beacuse it is
    ;; not possible to cancel finalization
  