mutating a hashmap in a struct

main.rs

use std::collections::HashMap;
#[derive(Debug)]
pub struct Element {
  pub a: u8,
}
#[derive(Debug)]
pub struct HSS {
  pub store: HashMap<u32, Vec<Element>>,
}
fn main() {
  let ve = vec![Element{a:1}, Element{a:2}];
  let mut hm = HashMap::new();
  hm.insert(42, ve);
  let mut hss = HSS { store: hm };
  hss.store.insert(19, vec![Element{a:3}]);
  println!("{:?}", hss);
}
// > ./main
// HSS { store: {42: [Element { a: 1 }, Element { a: 2}], 19: [Element { a: 3 }]} }