anchor.diff:      (setf slot-name (slot-definition-name slot-name)))
     (multiple-value-bind (slots layout)
         (fetch-up-to-date-instance-slots-and-layout instance)
  -    (let* ((location (or (fast-sv-position slot-name layout)
  -                         (error "The slot ~S is missing from the class ~S."
  -                                slot-name (class-of instance)))))
  -      (values slots location))))
  +    (let ((location (fast-sv-position slot-name layout)))
  +      (cond (location
  +             (values slots location))
  +            (t
  +             ;; Unknown slot, fall back on SLOT-LOCATION.
  +             (values slots
  +                     (slot-location (class-of instance) slot-name)))))))
   
   (defun std-slot-value (instance slot-name)
     (multiple-value-bind (slots location)
  
positive.code:       :initargs (remove-duplicates
                    (mapappend #'slot-definition-initargs
                               direct-slots))
        :allocation (slot-definition-allocation (car direct-slots)))))
  
  ;;;
  ;;; Generic function metaobjects and standard-generic-function
  
negative.code:     (setf (getf* slot ':readers) readers)
      (setf (getf* slot ':writers) writers)
      (setf (getf* slot ':allocation) allocation)
      (when (eql allocation :class)
        (setf (getf* slot ':location) (cons name secret-unbound-value)))
      (make-standard-slot-definition :properties slot)))
  
  (defun make-effective-slot-definition
  