fn main() {
    use std::collections::BTreeSet;

    let previous : BTreeSet<_> = [1, 3, 5].iter().cloned().collect();

    let newest : BTreeSet<_> = [3, 9, 11].iter().cloned().collect();;

    let removed: Vec<_> = previous.difference(&newest).cloned().collect();
    assert_eq!(removed, [1, 5]);

    let added: Vec<_> = newest.difference(&previous).cloned().collect();
    assert_eq!(added, [9, 11]);
}