#![allow(unused_variables)]
#![feature(btree_set_entry)]
fn main() {
    use std::collections::BTreeSet;
    let mut set = BTreeSet::from([1, 2, 3]);
    assert_eq!(set.len(), 3);
    assert_eq!(set.get_or_insert(2), & 2);
    assert_eq!(set.get_or_insert(100), & 100);
    assert_eq!(set.len(), 4);
}