ockquote>

<!-- So, that’s the real definition of ‘immutability’: is this safe to have two -->

<!-- pointers to? In `Arc<T>`’s case, yes: the mutation is entirely contained inside -->

<!-- the structure itself. It’s not user facing. For this reason, it hands out `&T` -->

<!-- with `clone()`. If it handed out `&mut T`s, though, that would be a problem. -->

<p>つまり、「イミュータビリティ」の真の定義はこうです: これは2箇所から指されても安全ですか?
<code>