$ cargo run
   Compiling cons-list v0.1.0 (file:///projects/cons-list)
    Finished dev [unoptimized + debuginfo] target(s) in 0.63s
     Running `target/debug/cons-list`
a after = Cons(RefCell { value: 15 }, Nil)
b after = Cons(RefCell { value: 3 }, Cons(RefCell { value: 15 }, Nil))
c after = Cons(RefCell { value: 4 }, Cons(RefCell { value: 15 }, Nil))
