% A Bad but Safe Doubly-Linked Deque

Now that we've seen Rc and heard about interior mutability, this gives an interesting thought... maybe we can mutate through an Rc. And it that's the case, maybe we can implement a doubly linked list totally safely!

In the process we'll become familiar with interior mutability, and probably learn the hard way that safe doesn't mean correct. Doubly linked lists are hard, and I always make a mistake somewhere.

Let's add a new file called fourth.rs:

// in lib.rs

pub mod first;
pub mod second;
pub mod third;
pub mod fourth;

This will be another clean-room operation, though as usual we'll probably find some logic that applies verbatim again.