 : chr  <span class="hljs-string">"1"</span> <span class="hljs-string">"10"</span> <span class="hljs-string">"100"</span> <span class="hljs-string">"1000"</span> <span class="hljs-keyword">...</span>
 $ topic             : int  <span class="hljs-literal">NA</span> <span class="hljs-literal">NA</span> <span class="hljs-literal">NA</span> <span class="hljs-number">3</span> <span class="hljs-literal">NA</span> <span class="hljs-number">4</span> <span class="hljs-number">2</span> <span class="hljs-number">4</sp