nk</span><span class="special">.</span><span class="identifier">clear</span><span class="special">();</span>
    <span class="special">};</span>

    <span class="comment">// Set the callback. The function requires a non-const reference so we</span>
    <span class="comment">// use a local variable, since temporaries can only bind to const refs.</span>
    <span class="identifier">p</span><span class="special">.</span><span class="identifier">on_chunk_header</span><span class="special">(</span><span class="