#![allow(unused_variables)]
#![feature(btree_set_entry)]
fn main() {
    use std::collections::BTreeSet;
    let mut set: BTreeSet<String> = ["cat", "dog", "horse"]
        .iter()
        .map(|&pet| pet.to_owned())
        .collect();
    assert_eq!(set.len(), 3);
    for &pet in &["cat", "dog", "fish"] {
        let value = set.get_or_insert_with(pet, str::to_owned);
        assert_eq!(value, pet);
    }
    assert_eq!(set.len(), 4);
}