How to use the contextualbandits.online._BasePolicy._get_drop_ix function in contextualbandits

----
        The available arms, if named, are stored in attribute 'choice_names'.
        
        Parameters
        ----------
        arm_name : int or object
            Arm to drop. If passing an integer, will drop at that index (starting at zero). Otherwise,
            will drop the arm matching this name (argument must be of the same type as the individual entries
            passed to 'nchoices' in the initialization).

        Returns
        -------
        self : object
            This object
        """
        drop_ix = _BasePolicy._get_drop_ix(self, arm_name)
        del self._oracles[drop_ix]
        _BasePolicy._drop_ix(self, drop_ix)
        return self