anchor.diff:            structure-class structure-object
             intern-eql-specializer eql-specializer eql-specializer-object
   
  +          slot-unbound
  +
             with-slots with-accessors
             ))
   
  
positive.code: (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))
  
negative.code:     (declare (ignore layout))
      (let* ((val (slot-contents slots location)))
        (if (eq secret-unbound-value val)
            (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)
  