a</span><span class=special>;
    </span><span class=identifier>subrule</span><span class=special>&lt;</span><span class=number>0</span><span class=special>&gt;  </span><span class=identifier>alias</span><span class=special>;  </span><span class=comment>// alias of a</span></code></pre>
<h3>Groups: scope and nesting</h3>
<p>The scope of a subrule and its definition is the enclosing group, typically 
  (and by convention) enclosed inside the parentheses. IDs outside a scope are 
  not directly visible. Inner