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

    let mut previous = BTreeSet::new();
    previous.insert(1);
    previous.insert(2);

    let mut newest = BTreeSet::new();
    newest.insert(2);
    newest.insert(3);

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

    let added: Vec<_> = newest.difference(&previous).cloned().collect();
    assert_eq!(added, [3]);
}