Your bug report

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

which was filed against the emacs package, has been closed.

The explanation is attached below, along with your original report.
If you require more details, please reply to 18786@debbugs.gnu.org.

-- 
18786: http://debbugs.gnu.org/cgi/bugreport.cgi?bug=18786
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