def phi_a(mass1, mass2, spin1x, spin1y, spin2x, spin2y):
    """ Returns the angle between the in-plane perpendicular spins."""
    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)
Returns the angle between the in-plane perpendicular spins.