 internal use only.</p>
<p>Apart from a few minor differences, the subrule follows the usage and syntax 
  of the rule closely. Here's the calculator grammar using subrules:</p>
<pre><code><font color="#000000"><span class=comment>    </span><span class=keyword>struct </span><span class=identifier>calculator </span><span class=special>: </span><span class=keyword>public </span><span class=identifier>grammar</span><span class=special>&lt;</span><span class=identifier>calculator</span><span class=special>&gt;