title: ERROR: Undefined procedure: (+)/2
body: <p>I'm new in SWI-Prolog and I want to check prime function in Prolog.</p>
  
  <pre><code>prime(N) :-
      N &gt; 1,
      M is N - 1,
      check(N, M).
  
  check(_, 1).
  check(N, M) :-
      M &gt; 1,
      R is N - M * N / M,
      R &gt; 0,
      P is M - 1,
      check(N, P).
  </code></pre>
  
  <p>but when I ask <code>2 + 2</code> or another operator like <code>(*)/2</code>, <code>(/)/2</code>,... in Prolog it says: <code>undefined procedure: (+)/2</code>.
  I think it is for <code>see</code> or <code>tell</code>. I reassign <code>see(user)</code> and <code>tell(user)</code> but this error occurs again.
  why?</p>