Non-recursively indicate that an iterable of _ReferenceKey no longer
        exist. Unknown keys are ignored.

        :param Iterable[_ReferenceKey] keys: The keys to drop.

def release_references(self, keys):
        """Non-recursively indicate that an iterable of _ReferenceKey no longer
        exist. Unknown keys are ignored.

        :param Iterable[_ReferenceKey] keys: The keys to drop.
        """
        keys = set(self.referenced_by) & set(keys)
        for key in keys:
            self.referenced_by.pop(key)