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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 url(self, node):
 """ return the url of a BeautifulSoup node or None if there is no url. """

        a_node = node.findParent('a')
        if not a_node:
            return None

        if self.link_callback is None:
            return a_node.get('href')
        else:
            return self.link_callback(a_node.get('href'))