係する値の強い参照カウントが0になれば、弱い参照が関わる循環はなんでも破壊されるので、
循環参照にはなりません。</p>
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Because the value that `Weak<T>` references might have been dropped, to do
anything with the value that a `Weak<T>` is pointing to, you must make sure the
value still exists. Do this by calling the `upgrade` method on a `Weak<T>`
instance, which will return an `Option<Rc<T>>`. You’ll get a result of `Some`
if the `Rc<T>` value has not