t>expression</tt> is the start subrule. When a group of subrules is 
  called forth, the start subrule <tt>expression</tt> is called first.</p>
<h3>IDs</h3>
<p>Each subrule has a corresponding ID; an integral constant that uniquely specifies 
  the subrule. Our example above has four subrules. They are declared as:</p>
<pre><code><span class=comment>    </span><span class=identifier>subrule</span><span class=special>&lt;</span><span class=number>0</span><span class=special>&gt;  </span><span class=identifie