    'disappeared' during attribute propagation. Notably, this is not only true
          for sequences but for any compound components. For instance, for alternative
          components the corresponding rule is:
        </p>
<pre class="programlisting"><span class="identifier">a</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">Unused</span> <span class="special">--&g