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