title: What does doxygen's $Id: ... $ mean
body: <p>I'm looking at a doxygen code example and seeing the following.  </p>
  
  <pre><code>/**  
    @file test.h  
    @brief Define helper functions  
    $Id: test.h,v 1.10 2012/10/10 10:10:10 author Exp $  
  */ 
  </code></pre>
  
  <p>Can anyone explain what does $Id: do here?</p>
  
  <p>Thanks</p>