     then the <code class="computeroutput"><span class="identifier">distinct</span></code> does
          not expose any attribute either.
        </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">B</span> <span class="special">--&gt;</span> <span class="identifier">distinct</span><span class="special">(</span>