an class="special">)</span> <span class="special">=</span> <span class="number">0</span><span class="special">;</span>     <span class="comment">// Pure virtual function.</span>
<span class="special">};</span>

<span class="identifier">TMP_ASSERT</span><span class="special">(</span>
    <span class="identifier">boost</span><span class="special">::</span><span class="identifier">remove_reference</span><span class="special">&lt;</span>            <span class="comment">// Add and remove</span>
        <span cl