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:     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>
    </dd>
  
    <dt><code>! { <em>predicate</em> }</code></dt>
  
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>
  