code: def get_extra_user_agents()
        @extra_user_agents_lock.synchronize do
          user_agents = @extra_user_agents.collect do |k, v|
            v.nil? ? k : '%s/%s' % [k, v]
          end
          @extra_user_agents.clear
          return user_agents
        end
      end
docs: Generates an array of extra user agents to include in the user agent
   string.