��1つの式からなるので、省略可能です。これらは全て、
呼び出された時に同じ振る舞いになる合法な定義です。</p>
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Closure definitions will have one concrete type inferred for each of their
parameters and for their return value. For instance, Listing 13-8 shows the
definition of a short closure that just returns the value it receives as a
parameter. This closure isn’t very useful except for the purposes of this
example. Note that we haven’t added any type an