anchor.diff:      (declare (ignore layout))
       (let* ((val (slot-contents slots location)))
         (if (eq secret-unbound-value val)
  -          (error "The slot ~S is unbound in the object ~S."
  -                 (slot-definition-name (elt (class-slots (class-of instance)) location))
  -                 instance)
  +          (slot-unbound (class-of instance)
  +                        instance
  +                        (slot-definition-name (elt (class-slots (class-of instance)) location)))
             val))))
   
   (defun fast-slot-write (new-value instance location)
  
positive.code:       (slot-location-in-instance instance slot-name)
      (let ((val (slot-contents slots location)))
        (if (eq secret-unbound-value val)
            (error "The slot ~S is unbound in the object ~S." slot-name instance)
            val))))
  (defun slot-value (object slot-name)
    (cond ((or (eq (class-of (class-of object)) the-class-standard-class)
  
negative.code:           structure-class structure-object
            intern-eql-specializer eql-specializer eql-specializer-object
  
            slot-unbound
  
            with-slots with-accessors
            ))
  
  