新的派生类应该能够替换基础类而无需修改代码。

The new derived classes should be able to replace the base classes without any change in the code.

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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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除了作为抽象类的派生类对象的一部分，是不能够生成抽象基类对象的。

We may not create objects of an abstract type except as parts of objects of classes derived from the abstract base.

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