anchor.diff:  ;;; Pane
   
   (defclass history-pane (application-pane)
  -  ((%state   :initarg  :state
  +  (;; The inspector state this history should observe and manipulate.
  +   (%state   :initarg  :state
                :reader   state)
      (%history :initarg  :history
                :reader   history
  
positive.code: 
  (define-command-table navigation-command-table)
  
  (define-command (com-select :command-table navigation-command-table
                              :name          "Inspect object")
      ((object inspected-object :gesture :describe))
    (let* ((frame   *application-frame*)
           (pane    (find-pane-named frame 'history))
           (state   (state pane)))
      (setf (root-object state :run-hook-p t) (object object)))) ; TODO getting the object can fail
  
  (define-command (com-visit :command-table navigation-command-table
                             :name          t)
      ((object element :gesture :select))
    (let* ((frame   *application-frame*)
           (pane    (find-pane-named frame 'history))
           (state   (state pane))
           (history (history pane)))
      (declare (ignore history))
      (setf (root-place state :run-hook-p t) object)))
  
  (define-command (com-back :command-table navigation-command-table
                            :name          t
                            :keystroke     (#\l :meta))
      ()
    (let* ((frame   *application-frame*)
           (pane    (find-pane-named frame 'inspector))
           (history (history (find-pane-named frame 'history))))
      (setf (root-place pane :run-hook-p t) (pop-element history))))
  
negative.code: (defmethod pop-element ((history history))
    (pop (elements history))
    (first (elements history)))
  
  (defmethod clear ((history history))
    (setf (elements history) '()))
  
  ;;; Presentations
  
  