title: context free grammar for a^n b^m
body: <p>I'm trying to make a CFG for the language:</p>
  
  <p><img src="https://i.stack.imgur.com/EiDgj.jpg" alt="enter image description here">  </p>
  
  <p>I did this:</p>
  
  <pre><code>S -&gt; a S b b 
  S -&gt; epsilon
  </code></pre>
  
  <p>It fulfills the requirement, but is it correct?</p>