              <span class="identifier">m</span> <span class="special">|</span>   <span class="identifier">m</span>   <span class="identifier">m</span>             <span class="identifier">M</span> <span class="special">|</span>     <span class="identifier">M</span> <span class="identifier">M</span>   <span class="identifier">M</span>
</pre>
<p>
          The second group can be viewed as an <span class="emphasis"><em>erasure by key objects</em></span>
</p>
<pre class="programlisting"><span class="comment">/