Undoing read-only region in emacs using ACL2

Question

I hit some strange incantation of keystrokes in emacs while using ACL2, and a region in my buffer becomes read-only. What can cause this? How do I unmark a region as read-only?

Answer 1

A Cause:

Suppose you have emacs-acl2.el loaded and the following in your ~/.emacs file:

(fset 'copy-and-advance
 [?\C-t ?\C-e return ?\C-x ?o ?\C-\M-f ?\C-\M-f ?\C-\M-b])

(global-set-key "\C-tn" 'copy-and-advance)
(global-set-key (kbd "<backtab>") 'copy-and-advance) ; backtab is Shift+tab

Then performing the following causes the problem:

<switch to *shell* buffer>
ctrl+x b
shift+tab
<"SwitchSwitch" now appears as a read-only region at the prompt of my *shell* buffer>

A solution:

Put this in your ~/.emacs file and reload your ~/.emacs file (note the credit to stack overflow question 7410125):

(defun set-region-writeable (begin end)
 "See http://stackoverflow.com/questions/7410125"
 (interactive "r")
 (let ((modified (buffer-modified-p))
       (inhibit-read-only t))
   (remove-text-properties begin end '(read-only t))
   (set-buffer-modified-p modified)))

Then execute the above function with:

ctrl-x h
meta-x set-region-writable
<enter>