code: def cas_a (freq_mhz, year):
      
      # The snu rule is right out of Baars et al. The dnu is corrected
      # for the frequency being measured in MHz, not GHz.
  
      snu = 10. ** (5.745 - 0.770 * np.log10 (freq_mhz)) # Jy
      dnu = 0.01 * (0.07 - 0.30 * np.log10 (freq_mhz)) # percent per yr.
      loss = (1 - dnu) ** (year - 1980.)
      return snu * loss
docs: Return the flux of Cas A given a frequency and the year of observation.
      Based on the formula given in Baars et al., 1977.
  
      Parameters:
  
      freq - Observation frequency in MHz.
      year - Year of observation. May be floating-point.
  
      Returns: s, flux in Jy.