Bug in `ed-previous-line' function in MCL 2.0b1p3

["David L. Westbrook" <Westy@COUNT.CS.UMASS.EDU>]

;; The following demonstrates a bug in `ed-previous-line' - a Fred
;; function which is supposed to move the cursor up one line. It fails.
;; The cursor moves up a line, but a subsequent call moves it down one line.

;; Eval this in the fred buffer to get *fred-window* bound to this window.
(defparameter *fred-window* (front-window :class 'fred-mixin))

;; Eval this and your cursor will incorrectly move back and forth.
(loop
   (ed-previous-line *fred-window*)
   (fred-update *fred-window*)
   (sleep .25))