pub trait OccupiedEntry<'a> {
type K;
type V;
// Required methods
fn key(&self) -> &Self::K;
fn get(&self) -> &Self::V;
fn get_mut(&mut self) -> &mut Self::V;
fn into_mut(self) -> &'a mut Self::V;
fn insert(&mut self, v: Self::V) -> Self::V;
fn remove(self) -> Self::V;
}