def secondary_xi(mass1, mass2, spin1x, spin1y, spin2x, spin2y):
    """Returns the effective precession spin argument for the smaller mass.
    """
    spinx = secondary_spin(mass1, mass2, spin1x, spin2x)
    spiny = secondary_spin(mass1, mass2, spin1y, spin2y)
    return xi2_from_mass1_mass2_spin2x_spin2y(mass1, mass2, spinx, spiny)
Returns the effective precession spin argument for the smaller mass.