How to use the pvlib.pvl_tools.sind function in pvlib

def aoi_projection(surf_tilt, surf_az, sun_zen, sun_az):
    """
    Calculates the dot product of the solar vector and the surface normal.
    
    Input all angles in degrees.
    
    :param surf_tilt: float or Series. Panel tilt from horizontal.
    :param surf_az: float or Series. Panel azimuth from north.
    :param sun_zen: float or Series. Solar zenith angle.
    :param sun_az: float or Series. Solar azimuth angle.
    
    :returns: float or Series. Dot product of panel normal and solar angle.
    """
    
    projection = pvl_tools.cosd(surf_tilt)*pvl_tools.cosd(sun_zen) + pvl_tools.sind(surf_tilt)*pvl_tools.sind(sun_zen)*pvl_tools.cosd(sun_az - surf_az)
    
    try:
        projection.name = 'aoi_projection'
    except AttributeError:
        pass
    
    return projection

# ratio of titled and horizontal beam irradiance
    # Will H: is this to avoid / 0? Shouldn't it be element-wise?
    #Rb = np.max(cos_tt, 0) / np.max(pvl_tools.cosd(sun_zen), 0.01745) 
    Rb = cos_tt / cos_sun_zen
    
    # Anisotropy Index
    AI = DNI / DNI_ET
    
    # DNI projected onto horizontal
    HB = DNI * cos_sun_zen   
    HB[HB < 0] = 0

    # these are actually the () and [] sub-terms of the second term of eqn 8
    term1 = 1 - AI
    term2 = 0.5 * (1 + pvl_tools.cosd(surf_tilt))
    term3 = 1 + np.sqrt(HB / GHI) * (pvl_tools.sind(0.5*surf_tilt) ** 3)

    sky_diffuse = DHI * ( AI*Rb + term1 * term2 * term3 )
    sky_diffuse[sky_diffuse < 0] = 0
    
    return sky_diffuse