an><span class="special">::</span><span class="identifier">string</span> <span class="special">&gt;,</span> <span class="comment">// author</span>
         <span class="identifier">set_of</span><span class="special">&lt;</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">string</span> <span class="special">&gt;,</span> <span class="comment">// title</span>

      <span class="identifier">with_info</span><span class="special">&lt;</span> <span class="identifier"