def _compute_scale(dimension, spread, special_scale):
    """See BFaS; p. 83.

    Parameters
    ----------
    dimension: int
        Spatial dimensionality of state space model
    spread: float
        Spread of sigma points around mean (1; alpha)
    special_scale: float
        Spread of sigma points around mean (2; kappa)

    Returns
    -------
    float
        Scaling parameter for unscented transform
    """
    return spread ** 2 * (dimension + special_scale) - dimension