use std::rc::Rc;

#[derive(Debug)]
enum List {
    Cons { n: i32, p: Rc<List> },
    Nil,
}
use List::{Cons, Nil};

fn main() {
    let nil = Rc::new(Nil);
    let mut a = Cons { n: 43, p: Rc::clone(&nil) };
    let mut b = Cons { n: 10, p: Rc::clone(&nil) };
    a.p = Rc::clone(&b);
    println!("{:?}", a);
}