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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 citedby_url(self):
 """URL to Scopus page listing citing papers."""

        cite_link = self.coredata.find('link[@rel="scopus-citedby"]', ns)
        try:
            return cite_link.get('href')
        except AttributeError:  # cite_link is None
            return None