match</span>
</pre>
<p>
            This form is usually used to trigger a semantic action unconditionally.
            For example, it is useful in triggering error messages when a set of
            alternatives fail:
          </p>
<pre class="programlisting"><span class="identifier">r</span> <span class="special">=</span> <span class="identifier">a</span> <span class="special">|</span> <span class="identifier">b</span> <span class="special">|</span> <span class="identifier">c</span> <span class="special