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{-# LANGUAGE FlexibleContexts #-}
-- This test comes from Sergei Mechveliani's DoCon system
module Pol3_ (moduloBasisx) where
class CommutativeRing a
class CommutativeRing a => LinSolvRing a
class LinSolvRing a => EuclideanRing a
instance EuclideanRing a => LinSolvRing (Pol a) -- XXXX
instance CommutativeRing a => CommutativeRing (Pol a)
data Pol a = MkPol
upLinSolvRing :: LinSolvRing a => a -> ()
upLinSolvRing = undefined
moduloBasisx :: (LinSolvRing (Pol a), CommutativeRing a) => Pol a -> ()
moduloBasisx p = let x = upLinSolvRing p
in ()
-- This is very delicate! The contraint (LinSolvRing (Pol a))
-- arises in the RHS of x, and we must be careful *not* to simplify
-- it with the instance declaration "XXXX", else we get the
-- unsatisfiable constraint (EuclideanRing a). In effect, the
-- given constraint in the type sig for moduleBasisx overlaps
-- with the top level declaration.
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