title: Does Prolog have an "if and only if" operator?
body: <p>I would normally write such a statement as two clauses, like this:</p>
  
  <pre><code>x :- y.
  y :- x.
  </code></pre>
  
  <p>In Prolog, is there any concise way to write a statement like "if x then y, and vice-versa"?</p>
  
  <p>In other words, is it possible to write "x if and only if y" as a single statement?</p>