[GHC] #9293: GHCI :unset -XLanguageExtension returns "don't know how to reverse -XLanguageExtension"

[GHC]

#9293: GHCI :unset -XLanguageExtension returns "don't know how to reverse
-XLanguageExtension"
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        Reporter:  jfeltz                        |            Owner:
            Type:  bug                           |           Status:  new
        Priority:  normal                        |        Milestone:
       Component:  GHCi                          |          Version:  7.8.2
      Resolution:                                |         Keywords:
Operating System:  Unknown/Multiple              |     Architecture:  x86
 Type of failure:  None/Unknown                  |       Difficulty:
       Test Case:  :set -XCPP followed by        |  Unknown
  :unset -XCPP                                   |       Blocked By:
        Blocking:                                |  Related Tickets:
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Comment (by nomeata):

 Is `:unset -XExt` the same as `:set -XNoExt`? Probably not for extensions
 that are on by default.

 This looks like a ticket suitable for new contributors to GHC.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/9293#comment:1>
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