anchor.diff:  (defun make-prompt-window (buffer parameters)
     (destructuring-bind (x y width height)
         (compute-window-rectangle buffer)
  -    (make-instance 'floating-prompt-window
  +    (make-instance 'floating-prompt
                      :buffer buffer
                      :x x
                      :y y
  
positive.code:     :initform nil
      :reader prompt-window-history)))
  
  (defclass floating-prompt-window (floating-window prompt-parameters)
    ((start-point
      :accessor prompt-window-start-charpos))
    (:default-initargs
  
negative.code:                    :called-window (prompt-window-called-window parameters)
                     :history (prompt-window-history parameters))))
  
  (defmethod lem::update-prompt-window ((window floating-prompt))
    (destructuring-bind (x y width height)
        (compute-window-rectangle (window-buffer window))
      (unless (and (= x (window-x window))
  