#![allow(unused_variables)]
fn main() {
    use std::collections::BTreeSet;
    let set = BTreeSet::from([3, 1, 2]);
    let mut set_iter = set.iter();
    assert_eq!(set_iter.next(), Some(& 1));
    assert_eq!(set_iter.next(), Some(& 2));
    assert_eq!(set_iter.next(), Some(& 3));
    assert_eq!(set_iter.next(), None);
}