title: Boolean assignment in Prolog
body: <p>all.</p>
  
  <p>I want to assign a boolean value to a variable.</p>
  
  <p>I've tried stuff like.</p>
  
  <pre><code>Diagonal is (XPiece = XFinal)
  Diagonal is (XPiece =:= XFinal)
  Diagonal is (XPiece is XFinal)
  </code></pre>
  
  <p>None work...</p>
  
  <p>Any solutions?</p>