src/Pure/Isar/term_style.ML

   val name = "Isar/antiquote_style";
   type T = ((Proof.context -> term -> term) * stamp) Symtab.table;
   val empty = Symtab.empty;
   val copy = I;
   val extend = I;
-  fun merge _ tabs = Symtab.merge eq_snd tabs
+  fun merge _ tabs = Symtab.merge (eq_snd (op =)) tabs
     handle Symtab.DUPS dups => err_dup_styles dups;
   fun print _ tab = Pretty.writeln (Pretty.strs ("antiquote styles:" :: Symtab.keys tab));
 end);
 
 val _ = Context.add_setup [StyleData.init];