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import Control.Monad (replicateM)
gray = map construct . (flip replicateM) ['0','1']
where construct x = zipWith (\a b -> if a == b then '0' else '1') ('0':x) x
gray' :: Integral a => a -> [String]
gray' 0 = [""]
gray' n = foldr (\s acc -> ("0" ++ s):("1" ++ s):acc) [] $ gray (n-1)
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