Generate OpenStreetMap element tree from ``Osm``.

def export_osm_file(self):
        """Generate OpenStreetMap element tree from ``Osm``."""
        osm = create_elem('osm', {'generator': self.generator,
                                  'version': self.version})
        osm.extend(obj.toosm() for obj in self)

        return etree.ElementTree(osm)