While it does not use this notation, I think this page is the right idea. https://en.wikipedia.org/w/index.php?title=Bag_(mathematics) https://en.wikipedia.org/

While it does not use this notation, I think this page is the right idea. https://en.wikipedia.org/w/index.php?title=Bag_(mathematics) https://en.wikipedia.org/w/index.php?title=Bag_(mathematics)