u may want to restrict a certain expression. You can think
        of a PEG expression as a match for a potentially infinite set of strings.
        The difference operator allows you to restrict this set:
      </p>
<pre class="programlisting"><span class="identifier">a</span> <span class="special">-</span> <span class="identifier">b</span>
</pre>
<p>
        The expression reads: match <code class="computeroutput"><span class="identifier">a</span></code>
        but not <code class="computeroutput"><span 