🚀 go-pugleaf

> [I get the error "Attempt to modify read-only object" when I attempt to
>  modify a cdr in auto-mode-alist.]

This is because the object exists in purespace in the dumped binary, which is
probably read-only.  Mumbo-jumbo.  This particular variable should be
user-mungeable.  As a quick fix, consider putting the following code in
site-init.el (or near the top of your .emacs if you aren't admin).

;; Make sure auto-mode-alist is modifiable.
(setq auto-mode-alist (copy-alist auto-mode-alist))

;; Function to update auto-modes.
(defun auto-mode-update (pattern function)
"Set the auto-mode function for filename regexp FILENAME-PATTERN to
function MODE-FUNCTION.  An auto-mode need not previously be defined for
FILENAME-PATTERN.  Returns the cons entry in auto-mode-alist."
  (interactive "sPattern: \nCFunction: ")
  (let ((entry (assoc pattern auto-mode-alist)))
    (if entry
      (setcdr entry function)
      (progn (setq entry (cons pattern function))
             (setq auto-mode-alist (cons entry auto-mode-alist))))
    entry))

Regards, [Ag]