def METHOD_NAME(self, destination: PackType, extended_pack: PackType):
    for _, yaml_file in extended_pack.list_files():
        yaml_file.ensure_deserialized(yaml.safe_load)
    self.rename_extra_files(extended_pack.extra)
    for namespace in extended_pack.values():
        self.rename_extra_files(namespace.extra)
    destination.merge(extended_pack)