n interval containers and
          a second part defining the <span class="emphasis"><em>*selection by key object</em></span>
          functionality.
        </p>
<p>
          
</p>
<pre class="programlisting"><span class="comment">/* (Generalized) intersection */</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"