<span class="caption">リスト15-27: 子供なしの<code>leaf</code>ノードと<code>leaf</code>を子要素に持つ<code>branch</code>ノードを作る</span></p>
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We clone the `Rc<Node>` in `leaf` and store that in `branch`, meaning the
`Node` in `leaf` now has two owners: `leaf` and `branch`. We can get from
`branch` to `leaf` through `branch.children`, but there’s no way to get from
`leaf` to `branch`. The reason is that `leaf` has no reference to `branch` and
doesn’t know they’re related