rhs_ty = head (dataConInstOrigArgTys con (mkTyVarTys tvs))
-- Instantiate the data con with the
-- type variables from the tycon
+ -- NB: a newtype DataCon has no existentials; hence the
+ -- call to dataConInstOrigArgTys has the right type args
etad_tvs :: [TyVar] -- Matched lazily, so that mkNewTypeCoercion can
etad_rhs :: Type -- return a TyCon without pulling on rhs_ty