code: def tau0_from_mtotal_eta(mtotal, eta, f_lower):
      r
      # convert to seconds
      mtotal = mtotal * lal.MTSUN_SI
      # formulae from arxiv.org:0706.4437
      return _a0(f_lower) / (mtotal**(5./3.) * eta)
docs: r"""Returns :math:`\tau_0` from the total mass, symmetric mass ratio, and
      the given frequency.