second element in the `Cons` variant is now `RefCell<Rc<List>>`, meaning that
instead of having the ability to modify the `i32` value as we did in Listing
15-24, we want to modify which `List` value a `Cons` variant is pointing to.
We're also adding a `tail` method to make it convenient for us to access the
second item if we have a `Cons` variant.
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<p>リスト15-5の<code>List</code>定義の別バリエーションを使用しています。<code>Cons</code>列挙子の2番目の要素はこれで<code