title: Prolog instantiation error?
body: <p>I'm stuck with some sort of error which i don't really understand in Prolog. I get this error when calling a rule(which seems to work sometimes?) :</p>
  
  <p><code>error(instantiation_error,Var0)</code></p>
  
  <p>Can anyone explain to me what this means?</p>
  
  <p>so i have two rules:</p>
  
  <p><code>special(X) :- user(X, Days), Days &gt;= 20.</code></p>
  
  <p><code>special(X) :- premiumuser(X).</code></p>
  
  <p><code>user(guy, 40).</code></p>
  
  <p>although user is added dynamically sometimes it is not in the Prolog db. I can eliminate the first special(x) rule and the error goes.</p>
  
  <p>when call call:</p>
  
  <p><code>special(someone).</code></p>
  
  <p>I get the error</p>