src/HOL/Tools/metis_tools.ML

       (*transform isabelle clause to metis clause *)
       fun add_thm is_conjecture (ith, {axioms, tfrees}) : logic_map =
         let val (mth, tfree_lits) = hol_thm_to_fol is_conjecture ctxt mode ith
         in
            {axioms = (mth, Meson.make_meta_clause ith) :: axioms,
-            tfrees = tfree_lits union tfrees}
+            tfrees = union (op =) (tfree_lits, tfrees)}
         end;
       val lmap0 = List.foldl (add_thm true)
                         {axioms = [], tfrees = init_tfrees ctxt} cls
       val lmap = List.foldl (add_thm false) (add_tfrees lmap0) ths
       val clause_lists = map (Metis.Thm.clause o #1) (#axioms lmap)