@staticmethod
def _validate_num_actions(num_actions):
    """Return num_actions if valid. Otherwise raise ValueError.

        Return
        ------
        int
            Number of possible actions.

        Raises
        ------
        ValueError
            If num_actions < 1

        """
    if num_actions < 1:
        raise ValueError('num_actions must be >= 1')
    return num_actions
