// We do a clone, and call `call` on that one. This makes sure that it's not our `RefCell` that complains about two mutable borrows,
// but rather the `RefCell` inside the `CallbacksMut`.
let mut c2: Callbacks = c.borrow().clone();
// We do a clone, and call `call` on that one. This makes sure that it's not our `RefCell` that complains about two mutable borrows,
// but rather the `RefCell` inside the `CallbacksMut`.
let mut c2: Callbacks = c.borrow().clone();