/p>
<pre class="programlisting"><span class="keyword">class</span> <span class="identifier">u</span> <span class="special">{</span>
    <span class="keyword">friend</span> <span class="keyword">class</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">contract</span><span class="special">::</span><span class="identifier">access</span><span class="special">;</span>

    <span class="keyword">void</span> <span class="identifier">invariant</span><span class="spec