lookupTyVar :: TvSubst -> TyVar  -> Maybe Type
        -- See Note [Extending the TvSubst] in TypeRep
lookupTyVar (TvSubst _ tenv) tv = lookupVarEnv tenv tv
lookupTyVar :: TvSubst -> TyVar  -> Maybe Type
lookupTyVar (TvSubst _ tenv) tv = lookupVarEnv tenv tv
lookupTyVar (TvSubst _ tenv) tv = lookupVarEnv tenv tv