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.

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定义了模式之后，我们现在可以创建一个关系型DAS的实例。

Having defined the model, we can now create an instance of the Relational DAS.

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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.

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