title: SWISH Prolog Errors in Factorial
body: <pre><code>fact(0,1).
  fact(N,F):-
      (   
          N&gt;0:-
          (   N1 is N-1,
              fact(N1,f1),
              F is N*F1
          )
      ;   
      write(&quot;N less than 0 nor allowed&quot;)
      ).
  </code></pre>
  <p>In the tutorial, the authors code is working. I have written the same code but mine is not working. He is writing in cgwin (i.e. offline) and I am writing in SWISH (i.e. online).</p>
  <p>Query passed: <code>fact(3,R).</code><br />
  Error message:</p>
  <pre><code>Singleton variables: [R]
  Singleton variables: [F1]
  procedure `(A:-B)' does not exist
  Reachable from:
        fact(A,B)
  </code></pre>