title: No permission to redefine imported_procedure
body: <p>I'm trying to create a predicate called <code>vertices/2</code> defined as:</p>
  
  <pre><code>vertices(G, Vs):- findall(V, vertex(G, V), Vs).
  </code></pre>
  
  <p>When I consult my file prolog gives me the error: </p>
  
  <blockquote>
    <p>No permission to redefine imported_procedure `ugraph:vertices/2' </p>
  </blockquote>
  
  <p>How can I avoid the redefinition?</p>
  
  <p>Thanks</p>