};
</span><span class="boring">use std::rc::Rc;
</span><span class="boring">use std::cell::RefCell;
</span><span class="boring">#[derive(Debug)]
</span><span class="boring">enum List {
</span><span class="boring">    Cons(i32, RefCell&lt;Rc&lt;List&gt;&gt;),
</span><span class="boring">    Nil,
</span><span class="boring">}
</span><span class="boring">
</span><span class="boring">impl List {
</span><span class="boring">    fn tail(&amp;self) -&gt; Option&lt;&amp;RefCell&lt;Rc&lt;List&gt;&gt;&gt; {
</span><s