title: How to express a transitive relationship
body: <p>I want to express a transitive relationship.  If A references B and B references C then A references C.  I have this:</p>
  
  <pre><code>proj(A).
  proj(B).
  proj(C).
  ref(A,B).
  ref(B,C).
  </code></pre>
  
  <p>When I query using <code>proj(A)</code> I obtain:</p>
  
  <blockquote>
    <p>[46] ?-proj(A).<br />
    A = _639</p>
  </blockquote>
  
  <p>What does "_639" mean?  I expected a yes or no and got that strangeness.  I need to add a rule to say:</p>
  
  <p><code>ref(A,C).</code> and get YES.  Ideally, if possible, I would like to show how this relationship came about: (A => B => C).</p>