interface Object{
  void accept(ObjectVisitor v);
}
interface Player extends Object{......}
interface ObjectVisitor{
  void visitPlayer(Player p);
  void visitNPC(NPC npc);
  ......
}
interface Action{
  void accept(ActionVisitor v);
}
interface ActionVisitor{
  void visitTalk(Player p, Object target);
  void visitGive(Player p, Object target);
  ......//all other actions
}
class GameEngine implements ActionVisitor, GameState{
  public void visitTalk(Player p, Object target){
    try{
      final Task t = findTaskForTalk(p, target);
      for(Event event: t.getEvents()){
        if(!event.complete(this, p)){
          failEvent(p, event);
        }
      }
      succeedTask(p, t);
    }
    catch(NoTaskException e){}
  }
  public void visitGive(Player p, Object target){
    ......
  }
  ...
}