gistfile1.hs

-- A.hs
module A where
data U = X | Y deriving (Eq, Show)

-- B.hs
module B where
import Data.Set
import A

instance Ord U where
compare X X = EQ
compare X Y = LT
compare Y X = GT
compare Y Y = EQ

ins :: U -> Set U -> Set U
ins = insert

-- C.hs
module C where
import Data.Set
import A

instance Ord U where
compare X X = EQ
compare X Y = GT
compare Y X = LT
compare Y Y = EQ

ins' :: U -> Set U -> Set U
ins' = insert

-- D.hs
module Main where
import Data.Set
import A
import B
import C

test :: Set U
test = ins' X $ ins X $ ins Y $ empty

main :: IO ()
main = print test