title: What does "fragment" mean in ANTLR?
body: <p>What does <em>fragment</em> mean in ANTLR?</p>
  
  <p>I've seen both rules:</p>
  
  <pre><code>fragment DIGIT : '0'..'9';
  </code></pre>
  
  <p>and </p>
  
  <pre><code>DIGIT : '0'..'9';
  </code></pre>
  
  <p>What is the difference?</p>