title: prolog trace how to use
body: <p>How to go second step when trace prolog program? 
  For example, I want to trace following simple program: </p>
  
  <pre><code>length1([],0).
  length1([_X|Xs],N):- length1(Xs,N1), N is N1+1.
  </code></pre>
  
  <p>I trace program: </p>
  
  <pre><code>?- trace,length([1,2,3],N).
  Call: (7) length([1, 2, 3], _G231) ? 
  Exit: (7) length([1, 2, 3], 3) ? creep
  N = 3.
  </code></pre>
  
  <p>But as we see, it immediately gives answer. But I thought it should be like <code>Call:(8) ... Call:(9) ...</code> What am I doing wrong?  </p>