anchor.diff:  (defun std-slot-value (instance slot-name)
     (multiple-value-bind (slots location)
         (slot-location-in-instance instance slot-name)
  +    (when (not location)
  +      (return-from std-slot-value
  +        (values (slot-missing (class-of instance) instance
  +                              slot-name 'slot-value))))
       (let ((val (slot-contents slots location)))
         (if (eq *secret-unbound-value* val)
             (values (slot-unbound (class-of instance) instance slot-name))
  
positive.code: (defun std-slot-boundp (instance slot-name)
    (multiple-value-bind (slots location)
        (slot-location-in-instance instance slot-name)
      (not (eq *secret-unbound-value* (slot-contents slots location)))))
  (defun slot-boundp (object slot-name)
    (let ((metaclass (class-of (class-of object))))
  
negative.code: 
  (defun slot-exists-p (instance slot-name)
    (not (null (find-effective-slot instance slot-name))))
  
  (defun slot-exists-in-class-p (class slot-name)
    (not (null (find slot-name (class-slots class)
                     :key #'slot-definition-name))))
  
  ;;; class-of
  
  