code: def chi_perp_from_mass1_mass2_xi2(mass1, mass2, xi2):
      
      q = q_from_mass1_mass2(mass1, mass2)
      a1 = 2 + 3 * q / 2
      a2 = 2 + 3 / (2 * q)
      return q**2 * a2 / a1 * xi2
docs: Returns the in-plane spin from mass1, mass2, and xi2 for the
      secondary mass.