书中epub版P335，Note的一句话说的比较困扰：
Ruby implements extend in exactly the same way, except the included class becomes the superclass of the target class’s class, or metaclass. Thus, extend allows you to add class methods to a class.”
后来经过和作者的沟通， 确定这是作者的一个错误：
详细来龙去脉可以参考这篇blog： http://tao.logdown.com/posts/240958-book-error-correcting-ruby-under-a-microscope