Currently, we have ``` haskell instance GNFData V1 where grnf = error "Control.DeepSeq.rnf: uninhabited type" ``` For old GHC, we should have ``` haskell grnf x = x `pseq` error "Control.DeepSeq.rnf: uninhabited type" ``` For newer GHC, we should have ``` haskell grnf x = case x of {} ```