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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 save(self):
 """Save xml to file."""

        if self.rater is not None:
            self.rater.set('modified', datetime.now().isoformat())

        xml = parseString(tostring(self.root))
        with open(self.xml_file, 'w') as f:
            f.write(xml.toxml())