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