title: Extend module inside another module
body: <p>I want to invoke method <code>a</code> from method of module <code>B</code>. How can I do it? I don't want to specify <code>A::a</code> every time.</p>
  
  <pre><code>module A
    def self.a
      "a"
    end
  end
  
  module B
    extend A
  
    def self.b
      a
    end
  end
  
  p B::b  # =&gt; undefined local variable or method `a' for B:Module
  </code></pre>