 subrule groups can be nested by enclosing each sub-group 
  inside another set of parentheses. Each group is unique and acts independently. 
  Consequently, while it may not be advisable to do so, a subrule in a group may 
  share the same ID as a subrule in another group since both groups are independent 
  of each other.</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=i