docs: 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)