anchor.diff:      ;; skip a prefix indicated by start
       (loop until (zerop start-bis)
   	  until (endp remaining)
  -	  do (setf remaining (cdr remaining)))
  +	  do (setf remaining (cdr remaining))
  +	     (decf start-bis))
       (unless (zerop start-bis)
         (error 'invalid-start-index
   	     :datum start
  
positive.code:     ;; skip a prefix indicated by start
      (loop until (zerop start-bis)
  	  until (endp remaining)
  	  do (setf remaining (cdr remaining)))
      (unless (zerop start-bis)
        (error 'invalid-start-index
  	     :datum start
  
negative.code: 	  do (setf remaining (cdr remaining))))
    list)
  
  (defun |nsubstitute seq-type=list from-end=nil end=nil test-not=eq count=other key=identity|
      (newitem olditem list start count)
    (let ((remaining list)
  	(start-bis start))
      ;; skip a prefix indicated by start
      (loop until (zerop start-bis)
  	  until (endp remaining)
  	  do (setf remaining (cdr remaining))
  	     (decf start-bis))
      (unless (zerop start-bis)
        (error 'invalid-start-index
  	     :datum start
  