If fields make sense for one subclass but not the other, you can remove them from the superclass and push them down into the subclass they belong to.
如果一些字段对于某个子类有意义，但是对于其他子类没有意义，就可以把它们从超类中删除，并放到它们所属的子类中。
Qualified subclass name: name subclasses to communicate the similarities and differences with a superclass.
子类名称要合格：对子类的命名会表述出其与超类之间的相似和差异之处。
Every class you define is a subclass of owl: thing and a superclass of owl: Nothing.
您所定义的每个类都是owl： thing的子类，并且是owl： Nothing的超类。
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