docs: def _register_service_type(cls, subclass):
          """Registers subclass handlers of various service-type-specific service
             implementations. Look for classes decorated with
             @Folder._register_service_type for hints on how this works."""
          if hasattr(subclass, '__service_type__'):
              cls._service_type_mapping[subclass.__service_type__] = subclass
              if subclass.__service_type__:
                  setattr(subclass,
                          subclass.__service_type__,
                          property(lambda x: x))
          return subclass