code: def primary_xi(mass1, mass2, spin1x, spin1y, spin2x, spin2y):
      
      spinx = primary_spin(mass1, mass2, spin1x, spin2x)
      spiny = primary_spin(mass1, mass2, spin1y, spin2y)
      return chi_perp_from_spinx_spiny(spinx, spiny)
docs: Returns the effective precession spin argument for the larger mass.