+function set_mt.__add(set1, set2)
+ return union(set1, set2);
+end
+function set_mt.__sub(set1, set2)
+ return difference(set1, set2);
+end
+function set_mt.__div(set, func)
+ local new_set = new();
+ local items, new_items = set._items, new_set._items;
+ for item in pairs(items) do
+ local new_item = func(item);
+ if new_item ~= nil then
+ new_items[new_item] = true;
+ end
+ end
+ return new_set;
+end
+function set_mt.__eq(set1, set2)
+ 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
+
+return {
+ new = new;
+ union = union;
+ difference = difference;
+ intersection = intersection;
+ xor = xor;
+};