code: def calc_extensions(self, extensions=None, Y_agg=None):
          
  
          ext_list = list(self.get_extensions(data=False))
          extensions = extensions or ext_list
          if type(extensions) == str:
              extensions = [extensions]
  
          for ext_name in extensions:
              self.meta._add_modify(
                  'Calculating accounts for extension {}'.format(ext_name))
              ext = getattr(self, ext_name)
              ext.calc_system(x=self.x,
                              Y=self.Y,
                              L=self.L,
                              Y_agg=Y_agg,
                              population=self.population
                              )
          return self
docs: Calculates the extension and their accounts
  
          For the calculation, y is aggregated across specified y categories
          The method calls .calc_system of each extension (or these given in the
          extensions parameter)
  
          Parameters
          ----------
          extensions : list of strings, optional
              A list of key names of extensions which shall be calculated.
              Default: all dictionaries of IOSystem are assumed to be extensions
          Y_agg : pandas.DataFrame or np.array, optional
              The final demand aggregated (one category per country).  Can be
              used to restrict the calculation of CBA of a specific category
              (e.g. households). Default: y is aggregated over all categories