A metaclass, again, has a class description. 同样地，一个元类也有类描述。
In practice, then, we can do without a separate concept of metaclass. 那么，实际上不用一个单独的元类概念我们也能做到这一切。
Class descriptions with the same set of methods are the objects of a metaclass. 同一组方法的类描述就是元类的对象。
A metaclass as such is a class and, therefore, has a class description. 如此一个元类也是一个类，因此有自己的类描述。
More classes in the same metaclass can be created just by sending new() to the metaclass description. 通过传送'''new()'''方法给元类描述，就可以创建该元类下更多别的类。
For the implementation of respondsTo() a method is passed to a metaclass constructor as a triple selector/tag/method. respondTo()的实现是传入一个选择器、标记、方法组成的三元组作为元类构造函数的参数。
