//@ `set`, which overrides the content, only needs a *shared borrow* of the cell. The phenomenon of a type that permits mutation through
//@ shared borrows (i.e., mutation despite the possibility of aliasing) is called *interior mutability*. You can think
//@ of `set` changing only the *contents* of the cell, not its *identity*. In contrast, the kind of mutation we saw so far was
//@ `set`, which overrides the content, only needs a *shared borrow* of the cell. The phenomenon of a type that permits mutation through
//@ shared borrows (i.e., mutation despite the possibility of aliasing) is called *interior mutability*. You can think
//@ of `set` changing only the *contents* of the cell, not its *identity*. In contrast, the kind of mutation we saw so far was