title: Why does boost::variant not provide operator !=
body: <p>Given two identical <code>boost::variant</code> instances <strong><code>a</code></strong> and <strong><code>b</code></strong>, the expression <strong><code>( a == b )</code></strong> is permitted.</p>
  
  <p>However <strong><code>( a != b )</code></strong> seems to be undefined.  Why is this?</p>