anchor.diff:                         (buffer-minor-modes))
                    (keymap-find-keybind (mode-keymap (buffer-major-mode)) key)
                    (keymap-find-keybind *global-keymap* key))))
  -    (function-to-command cmd)))
  +    (function-to-command (or cmd 'undefined-key))))
  
positive.code: (define-key *global-keymap* "C-@" 'undefined-key)
  (define-key *global-keymap* "C-a" 'undefined-key)
  (define-key *global-keymap* "C-b" 'undefined-key)
  (define-key *global-keymap* "C-c" 'undefined-key)
  (define-key *global-keymap* "C-d" 'undefined-key)
  (define-key *global-keymap* "C-e" 'undefined-key)
  (define-key *global-keymap* "C-f" 'undefined-key)
  
negative.code:     (unless (and (kbd-p kbd)
                   (every #'characterp (kbd-list kbd)))
        (error "define-key: ~s is illegal key" key))
      (define-key-internal keymap kbd fun)))
  
  (defun define-key-internal (keymap kbd fun)
    (do ((rest (kbd-list kbd) (cdr rest)))
        ((null (cdr rest))
         (setf (gethash (car rest) (keymap-table keymap)) fun))
      (let ((k (car rest)))
        (let ((cmd (gethash k (keymap-table keymap))))
          (if (keymap-p cmd)
              (setf keymap cmd)
              (let ((new-keymap (make-keymap)))
                (setf (gethash k (keymap-table keymap))
                      new-keymap)
                (setf keymap new-keymap)))))))
  
  (defun kbd-to-string (key)
    (format nil "~{~A~^~}"
  