title: Error Undefined Procedure in Prolog
body: <p>I wrote a rule:</p>
  
  <pre><code>parent(georgie,jessy) :-
     child(jessy,georgio).
  </code></pre>
  
  <p>but, when I want to prove </p>
  
  <pre><code>?- parent(georgie,jessy).
  </code></pre>
  
  <p>I get an error:</p>
  
  <pre><code>parent/2: Undefined procedure: child/2
  </code></pre>
  
  <p>Please help. Thanks.</p>