code: def get_Y_categories(self, entries=None):
          
          possible_dataframes = ['Y', 'FY']
  
          for df in possible_dataframes:
              if (df in self.__dict__) and (getattr(self, df) is not None):
                  try:
                      ind = getattr(self, df).columns.get_level_values(
                          'category').unique()
                  except (AssertionError, KeyError):
                      ind = getattr(self, df).columns.get_level_values(
                          1).unique()
                  if entries:
                      if type(entries) is str:
                          entries = [entries]
                      ind = ind.tolist()
                      return [None if ee not in entries else ee for ee in ind]
                  else:
                      return ind
          else:
              logging.warn("No attributes available to get Y categories")
              return None
docs: Returns names of y cat. of the IOSystem as unique names in order
  
          Parameters
          ----------
          entries : List, optional
              If given, retuns an list with None for all values not in entries.
  
          Returns
          -------
          Index
              List of categories, None if no attribute to determine
              list is available