code: def load_seit_data(directory, frequency_file='frequencies.dat',
                     data_prefix='volt_', **kwargs):
      frequencies = np.loadtxt(directory + os.sep + frequency_file)
      data_files = sorted(glob(directory + os.sep + data_prefix + '*'))
      if frequencies.size != len(data_files):
          raise Exception(
              'number of frequencies does not match number of data files')
      data_list = []
      for frequency, filename in zip(frequencies, data_files):
          subdata = load_mod_file(filename)
          subdata['frequency'] = frequency
          data_list.append(subdata)
      df = pd.concat(data_list)
      return df, None, None