#![allow(unused_variables)]
fn main() {
    use std::collections::BTreeMap;
    let mut map: BTreeMap<&str, String> = BTreeMap::new();
    let s = "hoho".to_string();
    map.entry("poneyland").or_insert_with(|| s);
    assert_eq!(map["poneyland"], "hoho".to_string());
}