title: What does class_methods do in concerns?
body: <p>I am reading some codes which use the concerns in Rails 4.</p>
  
  <p>I read some articles to say, if we would like to include class methods
  using module <code>ClassMethods</code>, but the code I read using something like:</p>
  
  <pre><code>class_methods do
    def **** 
    end
  end
  </code></pre>