anchor.diff:  It generates a user-SUPER subclass of SUPER.
   It binds `*SUPER-class*' to this newly generated class.
   
  +Classes can be modes or a core class like `browser', `buffer', `minibuffer',
  +`window'.
  +
   The `%slot-default' variable is replaced by the slot initform.
   
   Example that sets some defaults for all buffers:
  
positive.code:                                                                  ,(cadr slot))
                                                                 ,initform))
                  else do
                    (log:warn "Undefined slot ~a in ~a" (car slot) super)))
         (setf ,configured-class ',name))))
  
negative.code: In the above, `%slot-default' will be substituted with the default value of
  `default-modes'.
  
  To discover the default value of a slot or all slots of a class, use the
  `describe-slot' or `describe-class' commands respectively.
  
  Since modes are classes with class variables (the `*MODE-class*'), the same applies.
  
  Example to get the `blocker-mode' command to use a new default hostlists:
  
  \(define-configuration next/blocker-mode:blocker-mode
    ((next/blocker-mode:hostlists (append (list *my-blocked-hosts*) %slot-default))))
  
  Since the above binds `next/blocker-mode:*blocker-mode-class*' to
  `user-blocker-mode', the `blocker-mode' command now toggles the new
  `user-blocker-mode' instead of `blocker-mode'."
  
    (let* ((name (intern (str:concat "USER-" (symbol-name super))))
           (configured-class)
           (super-variant (let ((super-variant (intern (str:concat (symbol-name *renderer-class*) "-" (symbol-name super)))))
                            ;; SUPER may not have a renderer variant, e.g. modes don't.
                            (if (find-class super-variant nil)
                                super-variant
                                super))))
      (unless (find-class super-variant nil)
        (error "define-configuration argument ~a is not a known class." super))
      (setf configured-class (intern (str:concat "*" (symbol-name super) "-CLASS*") (symbol-package super-variant)))
      `(progn
         (defclass ,name (,super-variant)
           ,(loop with super-class = (closer-mop:ensure-finalized (find-class super-variant))
                  for slot in (car slots)
                  for known-slot? = (find (car slot) (mopu:slot-names super-class))
                  for initform = (and known-slot?
  