>のリストを指す別の<code>List</code>値を保持する<code>Rc&lt;List&gt;</code>インスタンスを変数<code>b</code>に生成します。</p>
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We modify `a` so it points to `b` instead of `Nil`, creating a cycle. We
do that by using the `tail` method to get a reference to the
`RefCell<Rc<List>>` in `a`, which we put in the variable `link`. Then we use
the `borrow_mut` method on the `RefCell<Rc<List>>` to change the value inside
from an `Rc<List>` that holds a `Nil` value to the `Rc<List>`