title: no permission to modify static procedure (\= )/2 prolog error
body: <p>I was trying to write a rule to tell when is it possible for "A" to be the brother of "B" but I got one error and one warning the first error says :<code>no permission to modify static procedure (\= )/2</code> 
    and the warning says :<code>Singleton variables [A,B]</code>. I understand that the singleton variable means that variable is used only once, but I cant figure a way to fix my code however I don't understand the first error I'm a Prolog beginner so it might happen to be a silly mistake that I can't find.</p>
  
  <pre><code> brother(A,B):-
      parent(X,A),
      parent(X,B),
      male(A).
   A \= B.
  </code></pre>