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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def manage_crawler(self):
 """ Manages a normal crawler thread. When a crawler finished, it loads another one if there are still sites to crawl. """

        index = True
        self.number_of_active_crawlers += 1
        while not self.shutdown and index is not None:
            index = self.crawler_list.get_next_item()
            if index is None:
                self.number_of_active_crawlers -= 1
                break

            self.start_crawler(index)