title: Japanese Wikipedia categories DAG defined in "categorylinks" table has a cycle
body: <p>I have found a reference cycle in the DAG defined by "categorylinks" and "page" tables inside the japanese Wikipedia database.
  Is this a bug in the data?</p>
  
  <p>Page ids reference cycle:</p>
  
  <p><code>2904319 -&gt; 133683 -&gt; 988775 -&gt; 424676 -&gt; 2904319</code></p>
  
  <p><code>(行動 -&gt; 生活 -&gt; 人間関係 -&gt; コミュニケーション -&gt; 行動)</code></p>
  
  <p>I am considering only sub-categories (page_namespace = 14).
  In the Wiki Category documentation it explicitly states that:</p>
  
  <blockquote>
    <p>All categories (except root category 1) should be contained in at least one other category, and there should be no cycles (i.e. a category should not contain itself, directly or indirectly).</p>
  </blockquote>
  
  <p>Could the data be broken?
  Am I misunderstanding anything?</p>