��の出力でわかります:</p>
<pre><code class="language-text">leaf parent = None
</code></pre>
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When we create the `branch` node, it will also have a new `Weak<Node>`
reference in the `parent` field, because `branch` doesn’t have a parent node.
We still have `leaf` as one of the children of `branch`. Once we have the
`Node` instance in `branch`, we can modify `leaf` to give it a `Weak<Node>`
reference to its parent. We use the `borrow_mut` method on the
`RefCell<Weak<Node>>` in the `parent` fiel