同样，实例化泛型类的客户机类必须使用满足对构造函数存在所声明的约束的类来这样做。
Also, client classes that instantiate our generic class must do so with classes that meet the declared constraint on what constructors exist.
有时我们想限制可能出现的泛型类的类型实例化。
Sometimes we want to restrict the potential type instantiations of a generic class.
泛型类可被视为将它们的参数映射成新实例化的函数。
Generic classes can be viewed as functions mapping their arguments to new instantiations.
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