code: def mass1_from_tau0_tau3(tau0, tau3, f_lower):
      r
      mtotal = mtotal_from_tau0_tau3(tau0, tau3, f_lower)
      eta = eta_from_tau0_tau3(tau0, tau3, f_lower)
      return mass1_from_mtotal_eta(mtotal, eta)
docs: r"""Returns the primary mass from the given :math:`\tau_0, \tau_3`.