Alright, we've mastered the art of mutable singly-linked stacks.
Let's move from single ownership to shared ownership by writing a persistent immutable singly linked list. This will be exactly the list that functional programmers have come to know and love. You can get the head or the tail and put someone's head on someone else's tail... and... that's basically it. Immutability is a hell of a drug.
In the process we'll largely just become familiar with Rc and Arc, but this will set us up for the next list which will change the game.
Let's add a new file called third.rs:
// in lib.rs pub mod first; pub mod second; pub mod third;
No copy-pasta this time. This is a clean room operation.