Subject: bug#18786: (error "Attempt to modify read-only object" (("\\.txz\\'" . tar-mode))) on adding mode

From: Stefan Monnier <[email-masked]>
Date: Tue, 21 Oct 2014 12:39:40 -0400

>     (nconc auto-mode-alist '(("\\.ctl" . scheme-mode))))

Try
  (add-to-list 'auto-mode-alist '("\\.ctl\\'" . scheme-mode))

-- Stefan