
To make the child node aware of its parent, we need to add a `parent` field to
our `Node` struct definition. The trouble is in deciding what the type of
`parent` should be. We know it can’t contain an `Rc<T>`, because that would
create a reference cycle with `leaf.parent` pointing to `branch` and
`branch.children` pointing to `leaf`, which would cause their `strong_count`
values to never be 0.
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<p>子供に親の存在を気付かせるために、<code>Node</code>構造体定義に<code>parent</code>