src/HOL/Tools/BNF/bnf_def.ML

                 let val Type (@{type_name set}, [elemT]) = fastype_of set_app in
                   if elemT = A then set_app else mk_UNION set_app (build elemT)
                 end;
             in
               if null set_apps then HOLogic.mk_set A []
-              else Library.foldr1 mk_union (map recurse set_apps)
+              else Library.foldl1 mk_union (map recurse set_apps)
             end
           | _ => HOLogic.mk_set A []));
   in build end;
 
 fun map_flattened_map_args ctxt s map_args fs =