Finds the smallest integer value >=0 that is not in `labels`

        :return: Value that is not in the labels
        :rtype: int

def _get_unknown_value(self):
        """ Finds the smallest integer value >=0 that is not in `labels`

        :return: Value that is not in the labels
        :rtype: int
        """
        label_set = set(self.labels)
        value = 0
        while value in label_set:
            value += 1
        return value