// **Exercise 12.1**: Write some piece of code using only the available, public interface of `CallbacksMut` such that a reentrant call to a closure
// is happening, and the program aborts because the `RefCell` refuses to hand out a second mutable borrow of the closure's environment.
// **Exercise 12.1**: Write some piece of code using only the available, public interface of `CallbacksMut` such that a reentrant call to a closure
// is happening, and the program aborts because the `RefCell` refuses to hand out a second mutable borrow of the closure's environment.