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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 export_kml_file(self):
 """Generate KML element tree from ``Placemarks``. Returns: etree.ElementTree: KML element tree depicting ``Placemarks`` """

        kml = create_elem('kml')
        kml.Document = create_elem('Document')
        for place in sorted(self.values(), key=lambda x: x.name):
            kml.Document.append(place.tokml())

        return etree.ElementTree(kml)