anchor.diff:      advance the parser position; otherwise consider the match failed.</p>
   
       <p>The code inside the predicate has access to all variables and functions
  -    defined in the initializer at the beginning of the grammar.</p>
  +    defined in the initializer at the beginning of the grammar. Curly braces in
  +    the predicate code must be balanced.</p>
     </dd>
   
     <dt><code><em>label</em> : <em>expression</em></code></dt>
  
positive.code: JavaScript code in curly braces ("{" and "}"). This code is executed before the
  generated parser starts parsing. All variables and functions defined in the
  initializer are accessible in <a href="#action">rule actions</a> and <a
  href="#semantic-predicate">semantic predicates</a>.</p>
  
  <p class="center">
    <img src="/img/rule.png" width="742" height="100" alt="PEG.js grammar rule">
  
negative.code:     return <code>null</code> to indicate a match failure.</p>
  
      <p>The code inside the action has access to all variables and functions
      defined in the initializer at the beginning of the grammar. Curly braces in
      the action code must be balanced.</p>
    </dd>
  
    <dt><code><em>expression<sub>1</sub></em> / <em>expression<sub>2</sub></em> / ... / <em>expression<sub>n</sub></em></code></dt>
  