def _reward_cert(y_output: np.ndarray) -> float:
        """
        Compute `cert` reward value.

        :param y_output: Output of the victim classifier.
        :return: Reward value.
        """
        largests = np.partition(y_output.flatten(), -2)[-2:]
        reward = largests[1] - largests[0]

        return reward

        Compute `cert` reward value.

        :param y_output: Output of the victim classifier.
        :return: Reward value.
        
art/attacks/extraction/knockoff_nets.py