//@ Putting it all together, the story around mutation and ownership through references looks as follows: There are *unique* references,
//@ which - because of their exclusivity - are always safe to mutate through. And there are *shared* references, where the compiler cannot
//@ generally promise that mutation is safe. However, if extra circumstances guarantee that mutation *is* safe, then it can happen even
//@ Putting it all together, the story around mutation and ownership through references looks as follows: There are *unique* references,
//@ which - because of their exclusivity - are always safe to mutate through. And there are *shared* references, where the compiler cannot
//@ generally promise that mutation is safe. However, if extra circumstances guarantee that mutation *is* safe, then it can happen even
// ## `RefCell`
//@ As the next step in the evolution of `Callbacks`, we could try to solve this problem of mutability once and for all, by adding `Cell`
// ## `RefCell`
//@ As the next step in the evolution of `Callbacks`, we could try to solve this problem of mutability once and for all, by adding `Cell`