code: def AddClass(self, class_name, gtfs_class):
      
      if class_name in self._class_mapping:
        raise problems.DuplicateMapping(class_name)
      self._class_mapping[class_name] = gtfs_class
docs: Adds an entry to the list of known classes.
  
      Args:
          class_name: A string with name through which gtfs_class is to be made
                      accessible.
          gtfs_class: The class to be added.
      Raises:
          DuplicateMapping if class_name is already present in the class mapping.