src/HOL/Tools/BNF/bnf_fp_n2m.ML

         resTs (map (Binding.suffix_name ("_" ^ un_foldN)) bs) (([], []), (empty_cache, lthy))
         |>> map_prod rev rev
         |>> pair ss
       end;
     val ((ss, (un_folds, un_fold_defs0)), (cache, (lthy, raw_lthy))) = lthy
-      |> Local_Theory.open_target |> snd
+      |> Local_Theory.open_target
       |> Variable.add_fixes (mk_names n "s")
       |> mk_un_folds
       ||> apsnd (`(Local_Theory.close_target));
 
     val un_fold_defs = map (unfold_thms raw_lthy @{thms Let_const}) un_fold_defs0;