anchor.diff:                  :lines (make-array (max 0 height) :initial-element nil)
                   :old-lines (make-array (max 0 height) :initial-element nil)))
   
  -(define-interface screen-delete (screen)
  +(define-implementation screen-delete (screen)
     (charms/ll:delwin (screen-%scrwin screen))
     (when (screen-%modeline-scrwin screen)
       (charms/ll:delwin (screen-%modeline-scrwin screen))))
   
  -(define-interface screen-clear (screen)
  +(define-implementation screen-clear (screen)
     (screen-modify screen)
     (charms/ll:clearok (screen-%scrwin screen) 1)
     (when (screen-%modeline-scrwin screen)
       (charms/ll:clearok (screen-%modeline-scrwin screen) 1)))
   
  -(define-interface screen-erase (screen)
  +(define-implementation screen-erase (screen)
     (charms/ll:werase (screen-%scrwin screen)))
   
   (defun screen-height (screen)
     (length (screen-lines screen)))
   
  -(define-interface screen-modify (screen)
  +(define-implementation screen-modify (screen)
     (setf (screen-modified-p screen) t))
   
  -(define-interface screen-set-size (screen width height)
  +(define-implementation screen-set-size (screen width height)
     (screen-modify screen)
     (when (screen-%modeline-scrwin screen)
       (decf height))
  
positive.code:     (print-echoarea nil nil)
      (redraw-display)))
  
  (define-interface print-echoarea (string doupdate-p)
    (charms/ll:werase *echo-area-scrwin*)
    (unless (null string)
      (charms/ll:mvwaddstr *echo-area-scrwin* 0 0 string))
  
negative.code: 
  (defvar *display-background-mode* nil)
  
  (define-implementation display-background-mode ()
    (or *display-background-mode*
        (lem.term:background-mode)))
  
  (define-implementation set-display-background-mode (mode)
    (check-type mode (or (eql :light) (eql :dark) null))
    (setf *display-background-mode* mode))
  
  (define-implementation set-foreground (name)
    (or (term-set-foreground name)
        (error "Undefined color: ~A" name)))
  
  (define-implementation set-background (name)
    (or (term-set-background name)
        (error "Undefined color: ~A" name)))
  
  