use std::collections::HashMap;

#[derive(Debug)]
struct Child(String);

#[derive(Debug)]
struct Parent<'a> {
    s: HashMap<String, &'a Child>
}

impl<'a> Parent<'a> {
    fn new() -> Parent<'a> {
        Parent {
            s: HashMap::new()
        }
    }

    fn s_mut(&mut self) -> &mut HashMap<String, &'a Child> {
        &mut self.s
    }
}

fn main() {
    let c = Child("a".to_string());
    let mut p = Parent::new();
    p.s_mut().insert("k".to_string(), &c);
    println!("{:?}", p);
}