[GHC] #10735: Smooth out the differences between `compiler/utils/Pretty.hs` and `libraries/pretty`

[GHC]

#10735: Smooth out the differences between `compiler/utils/Pretty.hs` and
`libraries/pretty`
-------------------------------------+-------------------------------------
        Reporter:  thomie            |                   Owner:  thomie
            Type:  task              |                  Status:  patch
        Priority:  normal            |               Milestone:  7.12.1
       Component:  Compiler          |                 Version:  7.0.1
      Resolution:                    |                Keywords:
Operating System:  Unknown/Multiple  |            Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |               Test Case:
      Blocked By:                    |                Blocking:
 Related Tickets:  #1062, #1176,     |  Differential Revisions:
  #7666, #8809                       |  Phab:D1130, Phab:D1132
-------------------------------------+-------------------------------------

Comment (by Thomas Miedema <thomasmiedema@…>):

 In [changeset:"5d57087e314bd484dbe14958f9b422be3ac6641a/ghc" 5d57087/ghc]:
 {{{
 #!CommitTicketReference repository="ghc"
 revision="5d57087e314bd484dbe14958f9b422be3ac6641a"
 Pretty: fix a broken invariant (#10735)

 This is a backport of a bug fix from
 6cfbd0444981c074bae10a3cf72733bcb8597bef in libraries/pretty:

     Fix a broken invariant
     Patch from #694,  for the problem "empty is an identity for <> and $$"
 is
     currently broken by eg. isEmpty (empty<>empty)"
 }}}

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/10735#comment:17>
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