="special">::</span><span class="identifier">failed</span></code>
      and <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">spirit</span><span class="special">::</span><span class="identifier">qi</span><span class="special">::</span><span class="identifier">rule</span></code>?
      Then this FAQ entry might be for you.
    </p>
<p>
      <a href="http://www.boost.org/libs/mpl/index.html" target="_top">Boost.Mpl</a> implements
      a