Tweak installation of selected packages

README.org

@@ -276,7 +276,11 @@ the ~custom-file~ as [[info:emacs#Saving Customizations][saving customizations (
    '(kill-ring-max 300)
    '(package-archives '(("gnu" . "https://elpa.gnu.org/packages/")
                         ("nongnu" . "https://elpa.nongnu.org/nongnu/")
-                        ("melpa" . "https://melpa.org/packages/"))))
+                        ("melpa" . "https://melpa.org/packages/")))
+   ;; Pin those packages to GNU ELPA to get the info documentation.
+   '(package-pinned-packages '((consult .  "gnu")
+                               (marginalia .  "gnu")
+                               (vertico . "gnu"))))
 #+end_src
 
 #+caption[Customize the second set of Emacs variables]:
@@ -318,6 +322,7 @@ the ~custom-file~ as [[info:emacs#Saving Customizations][saving customizations (
        rainbow-mode               ; set background color to color string
        smartparens                ; smart editing of character pairs
        toml-mode                  ; Tom's Obvious Minimal Language mode
+       undo-tree                  ; more advanced yet simpler undo system
        vertico                    ; VERTical Interactive Completion
        wgrep                      ; open a writable grep buffer
        which-key                  ; on the fly key-binding help
@@ -366,7 +371,7 @@ archives:
 1. The [[https://elpa.gnu.org/][GNU Emacs Lisp Package Archive]]
 2. The [[https://elpa.nongnu.org/][NonGNU Emacs Lisp Package Archive]].
 3. The [[https://melpa.org/#/][Milkypostman’s Emacs Lisp Package Archive (MELPA)]].
-Finally, the [[https://github.com/quelpa/quelpa][quelpa]] tool allows to fetch code from any source and build a
+In addition, the [[https://github.com/quelpa/quelpa][quelpa]] tool allows to fetch code from any source and build a
 package on your computer before installation.
 
 The code in listing [[lst:install-selected-packages]] assumes that the package
@@ -374,7 +379,10 @@ system is in a *virgin* state in case the package [[https://github.com/emacscoll
 Refreshing the contents of available packages at least once is a requirement in
 order to be able to install and load any packages, hence also [[https://github.com/emacscollective/no-littering][no-littering]].
 
-The call src_emacs-lisp{(package-install-selected-packages)} checks the
+Then, it ensures installation of [[https://github.com/quelpa/quelpa][quelpa]] before ensuring installation of
+[[https://www.reddit.com/r/emacs/comments/l6rcqh/undotree_repositorys_new_home_gitlab/][undo-tree]].
+
+Finally, the call src_emacs-lisp{(package-install-selected-packages)} checks the
 installation status of all packages in src_emacs-lisp{package-selected-packages}
 and installs the missing packages after the user has agreed to its prompt.  In
 case of normal Emacs usage, src_emacs-lisp{(package-list-packages)} refreshes
@@ -389,6 +397,13 @@ the contents of packages and allows to update packages to the latest version.
     (package-install 'no-littering)
     (require 'no-littering))
 
+  (unless (package-installed-p 'quelpa)
+    (package-install 'quelpa))
+
+  (unless (package-installed-p 'undo-tree)
+    ;; Neither GNU ELPA, nor MELPA have the latest version.
+    (quelpa '(undo-tree :repo "tsc25/undo-tree" :fetcher gitlab)))
+
   (unless noninteractive
     (package-install-selected-packages))
 #+end_src