def ld_power2(mu, a):
    """
    Evaluate power-2 limb-darkening law 

    :param mu: cos of angle between surface normal and line of sight
    :param a: array or tuple [c, alpha]

    :returns:  1 - c * (1-mu**alpha)
    
    """
    c, alpha = a
    return 1 - c * (1 - mu ** alpha)
