     </p>
<p>
          
</p>
<pre class="programlisting"><span class="comment">/* Selection by key objects */</span>     <span class="special">&amp;=</span> <span class="special">|</span> <span class="identifier">e</span> <span class="identifier">b</span> <span class="identifier">s</span> <span class="identifier">m</span>            <span class="special">&amp;=</span> <span class="special">|</span> <span class="identifier">e</span> <span class="identifier">i</span> <span class="identifier">b</span> <span c