title: prolog how does a built in predicate get an existence_error?
body: <p>so I have a prolog assignment that has to be tested with search algorithms provided by the teacher but I keep getting this error:</p>
  <pre><code>uncaught exception: error(existence_error(procedure,op/4),findall/3)
  </code></pre>
  <p>which I don't understand because op/4 and findall/3 are built in predicates, right? So how could they not exist?</p>
  <p>Anyone has any advice on getting over this? Thanks</p>