he parent of `leaf` again, this time we’ll get a `Some` variant
holding `branch`: now `leaf` can access its parent! When we print `leaf`, we
also avoid the cycle that eventually ended in a stack overflow like we had in
Listing 15-26; the `Weak<Node>` references are printed as `(Weak)`:
-->
<p>再度<code>leaf</code>の親を出力すると、今度は<code>branch</code>を保持する<code>Some</code>列挙子が得られます: これで<code>leaf</code>が親にアクセスできるようになったの