anchor.diff:  (defun window-offset-view (window)
     (let ((vtop-linum (window-vtop-linum window))
           (nlines (- (window-nlines window)
  -                   (length (window-wrap-ylist window))))
  +                   (length (window-collect-wrap-ylist window))))
           (linum (window-cur-linum window)))
       (cond
        ((< #1=(+ vtop-linum nlines -2) linum)
  
positive.code:                        (cons (fat-substring str 0 i) acc))))))
      (f str nil)))
  
  
  (defvar *wrapping-fatstring* (make-fatstring "!" 0))
  
  
negative.code: (defun window-recenter (window)
    (setf (window-vtop-linum window)
          (window-cur-linum window))
    (window-scroll window (- (floor (window-nlines window) 2)))
    (window-scroll window (window-wrapping-offset window)))
  
  (defun window-scroll (window n)
    (incf (window-vtop-linum window) n)
  