def _register_instance(cls, obj):
        """Registers the given instance in the weakref dictionary.

        Args:
            obj: a :class:`Document`
        """
        raise NotImplementedError(
            "subclass must implement _register_instance()"
        )
Registers the given instance in the weakref dictionary.

        Args:
            obj: a :class:`Document`
        
fiftyone/core/singletons.py