code: def xi2_from_mass1_mass2_spin2x_spin2y(mass1, mass2, spin2x, spin2y):
      
      q = q_from_mass1_mass2(mass1, mass2)
      a1 = 2 + 3 * q / 2
      a2 = 2 + 3 / (2 * q)
      return a1 / (q**2 * a2) * chi_perp_from_spinx_spiny(spin2x, spin2y)
docs: Returns the effective precession spin argument for the smaller mass.
      This function assumes it's given spins of the secondary mass.