docs: def _entry_must_exist(df, k1, k2):
      """Evaluate key-subkey existence.
  
      Checks that the key-subkey combo exists in the
      configuration options.
      """
      count = df[(df['k1'] == k1) &
                 (df['k2'] == k2)].shape[0]
      if count == 0:
          raise NotRegisteredError(
              "Option {0}.{1} not registered".format(k1, k2))