#![allow(unused_variables)]
fn main() {
    use std::collections::BTreeMap;
    use std::collections::btree_map::Entry;
    let mut map: BTreeMap<&str, u32> = BTreeMap::new();
    if let Entry::Vacant(o) = map.entry("poneyland") {
        o.insert(37);
    }
    assert_eq!(map["poneyland"], 37);
}