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  <h2>Haskell mode</h2>
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  module UniquePerms (
      uniquePerms
      )
  where
  
  -- | Find all unique permutations of a list where there might be duplicates.
  uniquePerms :: (Eq a) => [a] -> [[a]]
  uniquePerms = permBag . makeBag
  
  -- | An unordered collection where duplicate values are allowed,
  -- but represented with a single value and a count.
  type Bag a = [(a, Int)]
  
  makeBag :: (Eq a) => [a] -> Bag a
  makeBag [] = []
  makeBag (a:as) = mix a $ makeBag as
    where
      mix a []                        = [(a,1)]
      mix a (bn@(b,n):bs) | a == b    = (b,n+1):bs
                          | otherwise = bn : mix a bs
  
  permBag :: Bag a -> [[a]]
  permBag [] = [[]]
  permBag bs = concatMap (\(f,cs) -> map (f:) $ permBag cs) . oneOfEach $ bs
    where
      oneOfEach [] = []
      oneOfEach (an@(a,n):bs) =
          let bs' = if n == 1 then bs else (a,n-1):bs
          in (a,bs') : mapSnd (an:) (oneOfEach bs)
      
      apSnd f (a,b) = (a, f b)
      mapSnd = map . apSnd
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