code: def retrieve_exposure_classes_lists(exposure_keywords):
      
      # return None in case exposure doesn't have classifications
      classification = exposure_keywords.get('classification')
      if not classification:
          return None
      # retrieve classes definitions
      exposure_classifications = definition(classification)
      return exposure_classifications['classes']
docs: Retrieve exposures classes.
  
      Only if the exposure has some classifications.
  
      :param exposure_keywords: exposure keywords
      :type exposure_keywords: dict
  
      :return: lists of classes used in the classifications.
      :rtype: list(dict)