title: What is #= in Prolog
body: <p>The operator <code>#=</code> is mentioned on some page, e.g. <a href="https://www.metalevel.at/prolog" rel="nofollow">https://www.metalevel.at/prolog</a> but not on most other pages, e.g.: <a href="http://www.swi-prolog.org/pldoc/man?section=operators" rel="nofollow">http://www.swi-prolog.org/pldoc/man?section=operators</a></p>
  
  <p>What does this operator mean?</p>