Check out the replies to this comment [1] with the same attempted counterexample. Your set isn't union closed because {1}U{2} = {1, 2} isn't in it (same for {1}

Check out the replies to this comment [1] with the same attempted counterexample. Your set isn't union closed because {1}U{2} = {1, 2} isn't in it (same for {1}U{3} etc.). [1] https://news.ycombinator.com/item?id=34236889#34240381 https://news.ycombinator.com/item?id=34236889#34240381