docs: def choice(self, queues, connection):
          """
          Chooses a random queue for messages to specified connection.
  
          @param queues: A C{dict} mapping queue name to queues (sets of frames) to which 
                          specified connection is subscribed.
          @type queues:  C{dict} of C{str} to C{set} of L{stompclient.frame.Frame}
  
          @param connection: The connection that is going to be delivered the frame(s).
          @type connection: L{coilmq.server.StompConnection}
  
          @return: A random queue destination or None if list is empty.
          @rtype: C{str}
          """
          if not queues:
              return None
          return random.choice(list(queues.keys()))