def listen(&block)
      return if @listeners.any?
      @listeners = @queues.map do |queue|
        QueueListener.new(queue)
      end

      @listeners.each do |listener|
        listener.listen(&block)
      end
    end
def listen(&block)
      return if @listeners.any?
      @listeners = @queues.map do |queue|
        QueueListener.new(queue)
      end

      @listeners.each do |listener|
        listener.listen(&block)
      end
    end
(see QueueListener#listen)