title: warning in prolog
body: <p>I wrote this predicate in prolog :</p>
  
  <pre><code>list([]).
  list([X|L]) :- list(L).
  </code></pre>
  
  <p>it works well, but I got this warning :</p>
  
  <pre><code>    **Warning: /Users/hw6.pl:2:  
             Singleton variables: [X]** % 
  </code></pre>
  
  <p>what I can do to avoid it ?</p>