code: Occurrence getDefaultOccurrence()
      {
          if (default_occurence != null)
              return default_occurence;
  
          if (parent_queue != null)
              return parent_queue.getDefaultOccurrence();
  
          return null;
      }
docs: Gets the Queues default Occurrence. Will ask the parent Queue
  to get its default Occurrence if this Queue doesn't have one.