+
+isAddrRep :: PrimRep -> Bool
+-- True of machine adddresses; these are the things that don't
+-- work across DLLs.
+-- The key point here is that VoidRep comes out False, so that
+-- a top level nullary GADT construtor is False for isDllConApp
+-- data T a where
+-- T1 :: T Int
+-- gives
+-- T1 :: forall a. (a~Int) -> T a
+-- and hence the top-level binding
+-- $WT1 :: T Int
+-- $WT1 = T1 Int (Coercion (Refl Int))
+-- The coercion argument here gets VoidRep
+isAddrRep AddrRep = True
+isAddrRep PtrRep = True
+isAddrRep _ = False
+