code: def phi_a(mass1, mass2, spin1x, spin1y, spin2x, spin2y):
      
      phi1 = phi_from_spinx_spiny(primary_spin(mass1, mass2, spin1x, spin2x),
                                  primary_spin(mass1, mass2, spin1y, spin2y))
      phi2 = phi_from_spinx_spiny(secondary_spin(mass1, mass2, spin1x, spin2x),
                                  secondary_spin(mass1, mass2, spin1y, spin2y))
      return (phi1 - phi2) % (2 * numpy.pi)
docs: Returns the angle between the in-plane perpendicular spins.