docs: public static final <T> Set<T> symmetricDifference(Collection<Set<T>> sets)
      {
          MapSet<T,Integer> mapSet = new HashMapSet<>();
          int index = 0;
          for (Set<T> set : sets)
          {
              for (T t : set)
              {
                  mapSet.add(t, index);
              }
              index++;
          }
          final Set<T> diff = new HashSet<>();
          mapSet.forEach((t,s)->{if (s.size() == 1) diff.add(t);});
          return diff;
      }