a mysterous bug with ghc type class (GHC 6.2)

I have the following code:

it inserts a label pair into list of list, which contains label pairs 
sharing the common label.

I run it with ghci -fglasglow-exts -fallow-undecidable-instances


*Test> ins a Nil
Cons (Cons (LAB A [],[]) Nil) Nil
*Test> ins a (ins a Nil)
Cons (Cons (LAB A [],[]) (Cons (LAB A [],[]) Nil)) Nil
*Test> ins b (ins a Nil)
Cons (Cons (LAB A [],[]) Nil) (Cons (Cons (LAB B [],[]) Nil) Nil)
*Test> ins a (ins b (ins a Nil))
Cons (Cons (LAB A [],[]) (Cons (LAB A [],[]) Nil)) (Cons (Cons (LAB B 
[],[]) Nil) Nil)
*Test> ins b (ins a (ins b (ins a Nil)))
ghc-6.2: panic! (the `impossible' happened, GHC version 6.2):
       nameModule zddIns1 {- v a3rC -}


another trial:


*Test> ins b Nil
Cons (Cons (LAB B [],[]) Nil) Nil
*Test> ins a (ins b Nil)
Cons (Cons (LAB B [],[]) Nil) (Cons (Cons (LAB A [],[]) Nil) Nil)
*Test> ins b (ins a (ins b Nil))
ghc-6.2: panic! (the `impossible' happened, GHC version 6.2):
       nameModule zddIns1 {- v a5Ix -}


it seems whenever I insert label "b" into a list that contains list of 
"a" and list of "b", it pops into error.



Regards,
Kenny LU Zhuo Ming




module Test where


data LAB l r = LAB l r deriving Show

data OR a b = OR a b deriving Show


data Cons x y = Cons x y deriving Show

data Nil = Nil deriving Show

data T = T

data F = F

data A = A deriving Show

data B = B deriving Show

data Zero = Zero

data Succ n = Succ n

a = ((LAB A []),[])

b = ((LAB B []),[])

-- insertion function
-- insert label pairs in the a list of list, each list contains a 
collection of
-- label pair that sharing the common label.


class Ins r l l' | r l -> l' where
   ins :: r -> l -> l'


instance Ins ((LAB l1 r1),r1') Nil (Cons (Cons ((LAB l1 r1),r1') Nil) 
Nil) where
   ins l Nil = (Cons (Cons l Nil) Nil)
  

instance ( L2N l1 n1
    , L2N l2 n2
    , EqR n1 n2 b
    , Ins1 ((LAB l1 r1),r1') (Cons (Cons ((LAB l2 r2),r2') rs) rs') b l
    ) => Ins ((LAB l1 r1),r1') (Cons (Cons ((LAB l2 r2),r2') rs) rs') l 
where
   ins ((LAB l1 r1),r1') (Cons (Cons ((LAB l2 r2),r2') rs) rs') = ins1 
((LAB l1 r1),r1') (Cons (Cons ((LAB l2 r2),r2') rs) rs') (eqR (l2n l1) 
(l2n l2))


class Ins1 r l b l' | r l b -> l' where
   ins1 :: r -> l -> b -> l'

instance Ins1 ((LAB l1 r1),r1') (Cons r rs) T (Cons (Cons ((LAB l1 
r1),r1') r) rs) where
                                          ins1 l (Cons r rs) _ = (Cons 
(Cons l r) rs)

instance ( Ins ((LAB l1 r1),r1') rs rs'
    ) => Ins1 ((LAB l1 r1),r1') (Cons r rs) F (Cons r rs') where
   ins1 l (Cons r rs) _ = (Cons r (ins l rs))

-- class for mapping label to number

class L2N l n | l -> n where
   l2n :: l -> n

instance L2N A Zero where
   l2n A = Zero

instance L2N B (Succ Zero) where
   l2n B = Succ Zero


-- class for comparing number type

class EqR n1 n2 b | n1 n2 -> b where
   eqR :: n1 -> n2 -> b

instance EqR Zero Zero T where
   eqR _ _ = T

instance EqR Zero (Succ n) F where
   eqR _ _ = F

instance EqR (Succ n) Zero F where
   eqR _ _ = F

instance (EqR n1 n2 b) => EqR (Succ n1) (Succ n2) b where
   eqR (Succ n1) (Succ n2) = eqR n1 n