def _track_tendril(self, tendril):
        """
        Adds the tendril to the set of tracked tendrils.
        """

        self.tendrils[tendril._tendril_key] = tendril

        # Also add to _tendrils
        self._tendrils.setdefault(tendril.proto, weakref.WeakValueDictionary())
        self._tendrils[tendril.proto][tendril._tendril_key] = tendril
Adds the tendril to the set of tracked tendrils.