e the core might also want to take advantage 
  of this policy based machanism. Abstract syntax tree generation, debuggers and 
  lexers come to mind.</p>
<p> There are three sets of policies that govern:</p>
<ul>
  <li>Iteration and filtering</li>
  <li>Recognition and matching</li>
  <li>Handling semantic actions</li>
</ul>
<a name="iteration_policy"></a>
<h2>iteration_policy</h2>
<p> Here are the default policies that govern iteration and filtering:</p>
<pre>
    <code><span class=keyword>struct </span><