anchor.diff:  
   (defun %setf-complement (p val)
     (if (or (atom p)
  -		  (%slot-value? p))
  +		  (and *setf-immediate-slot-value*
  +			   (%slot-value? p)))
   	  (progn
   		(if (member p *constants*)
   		    (%error "cannot set constant"))
  
positive.code:     (%error "arguments expected")
      (if (= 1 (length args))
        `(%%defunsetf ,(car args)) ; Keep for DEFUN argument name.
        `(progn ,@(%setf args)))))
  
  (defun (setf symbol-function) (fun sym)
    (%set-atom-fun sym fun))
  
negative.code: ;;;; Copyright (C) 2005-2008 Sven Klose <pixel@copei.de>
  ;;;;
  ;;;; SETF macro
  
  (defvar *setf-immediate-slot-value* nil)
  
  ;; Assign evaluated value of argument y to variable x.
  (defmacro set (x y)
  