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{-# LANGUAGE RankNTypes #-}
module T10390 where
class ApPair r where
apPair :: (forall a . (ApPair a, Num a) => Maybe a) -> Maybe r
instance (ApPair a, ApPair b) => ApPair (a,b) where
apPair x = apPair' x
apPair' :: (ApPair b, ApPair c)
=> (forall a . (Num a, ApPair a) => Maybe a) -> Maybe (b,c)
-- NB constraints in a different order to apPair
apPair' f = let (Just a) = apPair f
(Just b) = apPair f
in Just $ (a, b)
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