创建一个从预定义grammar类继承而来的派生类。

Create a derived class inherited from the predefined grammar class.

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某些情况下，此公共功能不够完整，无法生成一个可用的对象，因此每个派生类都需要单独定义所缺少的功能。

In some cases, this common functionality is not complete enough to make a usable object, and each derived class defines the missing functionality.

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隐藏和重写都在派生类继承基类时使用，并且都是用另外的元素重定义一个已声明的元素。

Shadowing and overriding are both used when a derived class inherits from a base class, and both redefine one declared element with another.

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