title: Prolog existence_error following Seven Languages in Seven Weeks
body: <p>I'm just following the book Seven Languages in Seven Weeks.</p>
  
  <p>I've installed gprolog in my Mac machine using command <code>port install gprolog-devel</code> and run first prolog code.</p>
  
  <pre><code>likes(wallace, cheese).
  likes(grommit, cheese).
  likes(wendolene, sheep).
  
  friend(X, Y) :- \+(X = Y), likes(X, Z), likes(Y, Z).
  </code></pre>
  
  <p>If I type <code>likes(wallace, cheese).</code> on prolog interpretor, I've got error </p>
  
  <blockquote>
    <p>uncaught exception: error(existence_error(procedure,likes/2),top_level/0).</p>
  </blockquote>
  
  <p>Prolog 1.3.1 could not be installed in my Mac, I'm using prolog 1.4.0.</p>