Advanced concepts like abstract relations, or relation inheritance, are also part of the RDL language, but they will not be covered in this article.
更深入的概念，如抽象关联，或者抽象继承，也是RDL语言的一部分，但是本文将不作介绍。
Inheritance by extension is a common object-oriented design and is easily expressed through showing abstract classes in UML.
通过扩展来进行继承是一种常见的面向对象设计，很容易通过在U ML中显示抽象类来表达这种继承。
Offering both an interface and an abstract class is redundant, but I find it convenient: in most cases, I only need the abstract class, but the interface enables multiple inheritance.
同时提供接口和抽象类是多余的，但我发现这样很方便：在大多数情况下，我只需要抽象类，但接口支持多继承性。
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