src/HOL/Nominal/nominal_inductive2.ML

       | _ => fold (add_binders thy i) ts bs)
     | (u, ts) => add_binders thy i u (fold (add_binders thy i) ts bs))
   | add_binders thy i (Abs (_, _, t)) bs = add_binders thy (i + 1) t bs
   | add_binders thy i _ bs = bs;
 
-fun mk_set T [] = Const ("{}", HOLogic.mk_setT T)
+fun mk_set T [] = Const (@{const_name Set.empty}, HOLogic.mk_setT T)
   | mk_set T (x :: xs) =
       Const ("insert", T --> HOLogic.mk_setT T --> HOLogic.mk_setT T) $ x $
         mk_set T xs;
 
 fun split_conj f names (Const ("op &", _) $ p $ q) _ = (case head_of p of