title: Drools rule that fires when 2 objects share an attribute
body: <p>So I'm just beginning to tinker with Drools and am enjoying it,tough the documentation(the bits I've found at least) is a bit deep end all over.</p>
  
  <p>I'm trying to create a rule that will fire when two objects share an attribute but can't seem to get the condition right. If I'm reading the documentation right, this should work:
  (Yes, I am using Magic:The Gathering rules as a base for playing around because I know them well)</p>
  
  <pre>
  rule "704.5j. If two or more planeswalkers that share a planeswalker type are on the battlefield, all are put into their owners' graveyards. This is called the 'planeswalker uniqueness rule'."
      when
          $c1 : Card (CurrentZone == ZoneType.Battlefield , Types.contains("Planeswalker") , $subtype : Types.get(1) , $c1ID : ID );
          $c2 : Card (CurrentZone == ZoneType.Battlefield , Types.contains("Planeswalker") , Types.contains($subtype) , ID != $c1ID);
      then
          System.out.println("PW Uniqueness: " + $c1.getName() + " | " + $c2.getName());
          $c1.setCurrentZone(ZoneType.Graveyard);
          $c2.setCurrentZone(ZoneType.Graveyard);
  end
  </pre>
  
  <p>Will I have to do this on the java side of things?</p>
  
  <p>EDIT: Also, tutorial/guide suggestions for Drools are extremely welcome.</p>