docs: def subscriber_count(self, destination=None):
          """
          Returns a count of the number of subscribers.
  
          If destination is specified then it only returns count of subscribers 
          for that specific destination.
  
          @param destination: The optional topic/queue destination (e.g. '/queue/foo')
          @type destination: C{str}
          """
          if destination:
              return len(self._queues[destination])
          else:
              # total them up
              total = 0
              for k in self._queues.keys():
                  total += len(self._queues[k])
              return total