losures implement `FnOnce`,
because they can all be called at least once. Closures that don't move the
captured variables also implement `FnMut`, and closures that don't need mutable
access to the captured variables also implement `Fn`. In Listing 13-12, the
`equal_to_x` closure borrows `x` immutably (so `equal_to_x` has the `Fn` trait)
because the body of the closure only needs to read the value in `x`.
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<p>クロージャを生成する時、クロージャが環境を使用する方法に基づいて�