anchor.diff:  (defpackage :lem.listener-mode
     (:use :cl :lem :lem.util)
     (:export :listener-mode
  -           :*listener-mode-kemap*
  +           :*listener-mode-keymap*
  +           :listener-start-point
              :listener-start
              :listener-reset-prompt
              :listener-return
  
positive.code:   (unless (%listener-history)
      (setf (%listener-history)
            (make-history))))
  
  (defun listener-start (buffer-name mode)
    (let ((buffer (get-buffer-create buffer-name)))
  
negative.code: (define-command listener-prev-input () ()
    (multiple-value-bind (str win)
        (prev-history (%listener-history))
      (let ((start (listener-start-point))
            (end (point-max)))
        (delete-region start end)
        (when win (insert-string str))
  