 next item = Some(RefCell { value: Cons(5, RefCell { value: Nil }) })
b rc count after changing a = 2
a rc count after changing a = 2
</code></pre>
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The reference count of the `Rc<List>` instances in both `a` and `b` are 2
after we change the list in `a` to point to `b`. At the end of `main`, Rust
will try to drop `b` first, which will decrease the count in each of the
`Rc<List>` instances in `a` and `b` by 1.
-->
<p><code>a</code>のリストを<code>b</code>を指すように変更した後の<code>a<