Generate KML element tree from ``Placemarks``.

        Returns:
            etree.ElementTree: KML element tree depicting ``Placemarks``

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)