+local set_mt = {};
+function set_mt.__call(set, _, k)
+ return next(set._items, k);
+end
+function set_mt.__add(set1, set2)
+ return _M.union(set1, set2);
+end
+function set_mt.__sub(set1, set2)
+ return _M.difference(set1, set2);
+end
+function set_mt.__div(set, func)
+ local new_set, new_items = _M.new();
+ local items, new_items = set._items, new_set._items;
+ for item in pairs(items) do
+ if func(item) then
+ new_items[item] = true;
+ end
+ end
+ return new_set;
+end
+function set_mt.__eq(set1, set2)
+ local set1, set2 = set1._items, set2._items;
+ for item in pairs(set1) do
+ if not set2[item] then
+ return false;
+ end
+ end
+
+ for item in pairs(set2) do
+ if not set1[item] then
+ return false;
+ end
+ end
+
+ return true;
+end
+function set_mt.__tostring(set)
+ local s, items = { }, set._items;
+ for item in pairs(items) do
+ s[#s+1] = tostring(item);
+ end
+ return t_concat(s, ", ");
+end
+
+local items_mt = {};
+function items_mt.__call(items, _, k)
+ return next(items, k);
+end
+