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{-# LANGUAGE TypeFamilies, EmptyDataDecls #-}
module T2639 where
data Eps
data family Work a v
data instance Work Eps v = Eps v
type family Dual a
type instance Dual Eps = Eps
class Connect s where
connect :: (Dual s ~ c, Dual c ~ s) => Work s a -> Work c b -> (a,b)
instance Connect Eps where
connect (Eps a) (Eps b) = (a,b)
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