docs: @SafeVarargs
      private final <T> Set<T> join(Set<T>... sets)
      {
          Set<T> result = new HashSet<>();
          if (sets == null)
              return result;
  
          for (Set<T> set : sets)
          {
              if (set != null)
                  result.addAll(set);
          }
          return result;
      }