docs: public static Set<String> extractFCMTopics(final Criteria criteria, final String variantID) {
          final Set<String> topics = new TreeSet<>();
  
          if (isEmptyCriteria(criteria)) {
              // use the variant 'convenience' topic
              topics.add(Constants.TOPIC_PREFIX + variantID);
  
          } else if (isCategoryOnlyCriteria(criteria)) {
              // use the given categories
              topics.addAll(criteria.getCategories().stream()
                      .map(category -> Constants.TOPIC_PREFIX + category)
                      .collect(Collectors.toList()));
          }
          return topics;
      }