code: def eta_from_tau0_tau3(tau0, tau3, f_lower):
      r
      mtotal = mtotal_from_tau0_tau3(tau0, tau3, f_lower,
                                     in_seconds=True)
      eta = mtotal**(-2./3.) * (_a3(f_lower) / tau3)
      return eta
docs: r"""Returns symmetric mass ratio from :math:`\tau_0, \tau_3`.