Add sections on narrowing and disabled commands

README.org

@@ -935,6 +935,62 @@ option to run different commands on those selections.
 
 * Editing
 
+** [[https://www.emacswiki.org/emacs/DisabledCommands][Enable disabled commands and inform]]
+:PROPERTIES:
+:CUSTOM_ID: sec:enable-disabled-commands
+:END:
+
+Execute src_emacs-lisp[:exports code]{(find-library "novice")} to see how
+Emacs prevents new users from shooting themselves in the feet.
+
+#+begin_src emacs-lisp
+  (setq disabled-command-function
+        (defun my-enable-this-command (&rest _args)
+          "Called when a disabled command is executed.
+  Enable it and re-execute it."
+          (put this-command 'disabled nil)
+          (message "You typed %s.  %s was disabled until now."
+                   (key-description (this-command-keys)) this-command)
+          (sit-for 0)
+          (call-interactively this-command)))
+#+end_src
+
+
+** [[info:emacs#Narrowing][Narrowing]]
+:PROPERTIES:
+:CUSTOM_ID: sec:narrowing
+:END:
+
+[[https://endlessparentheses.com/emacs-narrow-or-widen-dwim.html][Emacs narrow-or-widen-dwim]]
+
+#+begin_src emacs-lisp
+  (defun narrow-or-widen-dwim (p)
+    "Widen if buffer is narrowed, narrow-dwim otherwise.
+  Dwim means: region, org-src-block, org-subtree, or defun,
+  whichever applies first. Narrowing to org-src-block actually
+  calls `org-edit-src-code'.
+  With prefix P, don't widen, just narrow even if buffer is
+  already narrowed."
+    (interactive "P")
+    (declare (interactive-only))
+    (cond ((and (buffer-narrowed-p) (not p)) (widen))
+          ((and (bound-and-true-p org-src-mode) (not p))
+           (org-edit-src-exit))
+          ((region-active-p)
+           (narrow-to-region (region-beginning) (region-end)))
+          ((derived-mode-p 'org-mode)
+           (or (ignore-errors (org-edit-src-code))
+               (ignore-errors (org-narrow-to-block))
+               (org-narrow-to-subtree)))
+          ((derived-mode-p 'latex-mode)
+           (LaTeX-narrow-to-environment))
+          ((derived-mode-p 'tex-mode)
+           (TeX-narrow-to-group))
+          (t (narrow-to-defun))))
+
+  (define-key ctl-x-map (kbd "C-n") #'narrow-or-widen-dwim)
+#+end_src
+
 ** Synchronal multiple-region editing
 
 #+attr_latex: :options bgcolor=LightGoldenrodYellow