anchor.diff:  (defmethod update-instance-for-redefined-class ((instance standard-object) added-slots discarded-slots property-list &rest initargs &key &allow-other-keys)
     (declare (ignore discarded-slots property-list))
     (apply #'shared-initialize instance added-slots initargs))
  +
  +(defgeneric slot-unbound (class instance slot-name))
  +
  +(defmethod slot-unbound ((class t) instance slot-name)
  +  (error "The slot ~S is unbound in the object ~S."
  +         slot-name
  +         instance))
  
positive.code:           structure-class structure-object
            intern-eql-specializer eql-specializer eql-specializer-object
  
            with-slots with-accessors
            ))
  
  
negative.code:       (slot-location-in-instance instance slot-name)
      (let ((val (slot-contents slots location)))
        (if (eq secret-unbound-value val)
            (slot-unbound (class-of instance) instance slot-name)
            val))))
  (defun slot-value (object slot-name)
    (cond ((or (eq (class-of (class-of object)) the-class-standard-class)
  