#![allow(unused_variables)]
fn main() {
    use std::collections::BTreeSet;
    let mut a = BTreeSet::new();
    a.insert(1);
    a.insert(2);
    let mut b = BTreeSet::new();
    b.insert(2);
    b.insert(3);
    let diff: Vec<_> = a.difference(&b).cloned().collect();
    assert_eq!(diff, [1]);
}