title: Doxygen requirements tracking / inverse matrix
body: <p>I use a custom Doxygen tag to track requirement implementations in my software source code, like so :</p>
  
  <pre><code>/// @req REQ-BLAH-0010
  </code></pre>
  
  <p>The @req tag is defined in my doxygen.conf file:</p>
  
  <pre><code>ALIASES += "req=\xrefitem req \"Requirement\" \"Requirements\" "
  </code></pre>
  
  <p>This works, Doxygen generates a list of requirements for each method/class/file. But sometimes the same requirement number is used in different places, and I'd like a way to get the inverse list: for each requirement number, list all the places in which it is tracked.</p>
  
  <p>Is this possible to do using Doxygen? If so, how? If not, what tool would you recommend for this?</p>