title: What kinds of header files should not be protected against multiple inclusion?
body: <p>I read the dcmtk source code, and found a comment in <a href="http://support.dcmtk.org/docs/ofstdinc_8h-source.html">ofstdinc.h</a>:</p>
  
  <pre><code>// this file is not and should not be protected against multiple inclusion
  </code></pre>
  
  <p>And what kinds of header files <strong>SHOULD NOT</strong> be protected against multiple inclusion?</p>