docs: def add_from_vla_obs (src, Lband, Cband):
      """Add an entry into the models table for a source based on L-band and
      C-band flux densities.
  
      """
      if src in models:
          raise PKError ('already have a model for ' + src)
  
      fL = np.log10 (1425)
      fC = np.log10 (4860)
  
      lL = np.log10 (Lband)
      lC = np.log10 (Cband)
  
      A = (lL - lC) / (fL - fC)
      B = lL - A * fL
  
      def fluxdens (freq_mhz):
          return 10. ** (A * np.log10 (freq_mhz) + B)
  
      def spindex (freq_mhz):
          return A
  
      models[src] = fluxdens
      spindexes[src] = spindex