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+{-
+  From: Marc van Dongen <dongen@cs.ucc.ie>
+  Date: Sat, 31 May 1997 19:57:46 +0100 (BST)
+
+   panic! (the `impossible' happened):
+           tcLookupTyVar:a_r6F
+
+   Please report it as a compiler bug to glasgow-haskell-bugs@dcs.gla.ac.uk.
+
+
+If the instance definition for (*) at the end of this toy module
+is replaced by the definition that is commented, this all compiles
+fine. Strange, because the two implementations are equivalent modulo
+the theory {(*) = multiply}.
+
+Remove the `multiply :: a -> a -> a' part, and it compiles without
+problems.
+
+
+SPJ note: the type signature on "multiply" should be
+	multiply :: Group a => a -> a -> a
+
+-}
+
+module ShouldSucceed( Group, Ring ) where
+
+import qualified Prelude( Ord(..), Eq(..), Num(..) )
+import Prelude hiding( Ord(..), Eq(..), Num(..) )
+
+class Group a where
+  compare     :: a -> a -> Prelude.Ordering
+  fromInteger :: Integer -> a
+  (+) :: a -> a -> a
+  (-) :: a -> a -> a
+  zero :: a
+  one  :: a
+  zero = fromInteger 0
+  one  = fromInteger 1
+
+-- class (Group a) => Ring a where
+-- (*) :: a -> a -> a
+-- (*) a b =
+--                  case (compare a zero) of
+--                    EQ -> zero
+--                    LT -> zero - ((*) (zero - a) b)
+--                    GT -> case compare a one of
+--                            EQ -> b
+--                            _  -> b + ((*) (a - one) b)
+
+class (Group a) => Ring a where
+  (*) :: a -> a -> a
+  (*) a b = multiply a b
+          where multiply :: Group b => b -> b -> b
+                multiply a b
+                  = case (compare a zero) of
+                      EQ -> zero
+                      LT -> zero - (multiply (zero - a) b)
+                      GT -> case compare a one of
+                              EQ -> b
+                              _  -> b + (multiply (a - one) b)