有时候看源码的时候看到类名之间存在<.<=操作，顿时一头雾水，类名之间也可以比较吗？查了下api，豁然开朗
Class的父类是Module，Module.methods.grep(/</)
会得到匹配的结果集：[:<=>,:<,:<=]
e.g
mod < other → true, false, or nil
Returns true if mod is a subclass of other. Returns nil if there’s no relationship between the two. (Think of the relationship in terms of the class definition: “class A<B” implies “A<B”).
mod <= other → true, false, or nil
Returns true if mod is a subclass of other or is the same as other. Returns nil if there’s no relationship between the two. (Think of the relationship in terms of the class definition: “class A<B” implies “A<B”).