// We have to *explicitly* borrow the contents of a `RefCell` by calling `borrow` or `borrow_mut`.
let mut closure = callback.borrow_mut();
// Unfortunately, Rust's auto-dereference of pointers is not clever enough here. We thus have to explicitly
// We have to *explicitly* borrow the contents of a `RefCell` by calling `borrow` or `borrow_mut`.
let mut closure = callback.borrow_mut();
// Unfortunately, Rust's auto-dereference of pointers is not clever enough here. We thus have to explicitly