code: private static List<List<String>> sortedTagUnion(@Nullable List<List<String>> existingTags, Set<String> newTagSet) {
          return sortedTagUnion(existingTags, asSortedList(newTagSet));
      }
docs: Produces a list-of-lists for all unique tag sets for a given databus event.  <code>existingTags</code> must either
  be null or the result of a previous call to {@link #sortedTagUnion(java.util.List, java.util.List)} or
  {@link #sortedTagUnion(java.util.List, java.util.Set)}.