code: def final_mass_from_f0_tau(f0, tau, l=2, m=2):
      
      # from Berti et al. 2006
      spin = final_spin_from_f0_tau(f0, tau, l=l, m=m)
      a, b, c = _berti_mass_constants[l,m]
      return (a + b*(1-spin)**c)/(2*numpy.pi*f0*lal.MTSUN_SI)
docs: Returns the final mass (in solar masses) based on the given frequency
      and damping time.
  
      .. note::
          Currently, only l = m = 2 is supported. Any other indices will raise
          a ``KeyError``.
  
      Parameters
      ----------
      f0 : float or array
          Frequency of the QNM (in Hz).
      tau : float or array
          Damping time of the QNM (in seconds).
      l : int, optional
          l-index of the harmonic. Default is 2.
      m : int, optional
          m-index of the harmonic. Default is 2.
  
      Returns
      -------
      float or array
          The mass of the final black hole. If the combination of frequency
          and damping times give an unphysical result, ``numpy.nan`` will be
          returned.