[GHC] #10344: Make BranchList simpler

[GHC]

#10344: Make BranchList simpler
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        Reporter:  goldfire          |                   Owner:  goldfire
            Type:  task              |                  Status:  new
        Priority:  normal            |               Milestone:
       Component:  Compiler          |                 Version:  7.11
      Resolution:                    |                Keywords:
Operating System:  Unknown/Multiple  |            Architecture:
 Type of failure:  None/Unknown      |  Unknown/Multiple
      Blocked By:                    |               Test Case:
 Related Tickets:                    |                Blocking:
                                     |  Differential Revisions:
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Comment (by simonpj):

 But he uses `Maybe (CoAxiom Branched)` because branch-lists could not
 previously represent empty lists.  Once they can we can get rid of the
 `Maybe`.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/10344#comment:4>
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