src/HOLCF/Tools/domain/domain_extender.ML

           in if Symbol.is_letter c then c else "t" end
       | typid (TFree (id,_)   ) = hd (strip (tl (Symbol.explode id)))
       | typid (TVar ((id,_),_)) = hd (tl (Symbol.explode id));
     fun one_con (con,args,mx) =
 	((Syntax.const_name mx (Binding.name_of con)),
-	 ListPair.map (fn ((lazy,sel,tp),vn) => ((lazy,
+	 ListPair.map (fn ((lazy,sel,tp),vn) => mk_arg ((lazy,
 					find_index_eq tp dts,
 					DatatypeAux.dtyp_of_typ new_dts tp),
 					Option.map Binding.name_of sel,vn))
 	     (args,(mk_var_names(map (typid o third) args)))
 	 ) : cons;