code: public static RecurrenceIterator join(
              RecurrenceIterator a, RecurrenceIterator... b) {
          List<RecurrenceIterator> incl = new ArrayList<RecurrenceIterator>();
          incl.add(a);
          incl.addAll(Arrays.asList(b));
          return new CompoundIteratorImpl(
                  incl, Collections.<RecurrenceIterator>emptyList());
      }
docs: a recurrence iterator that returns the union of the given recurrence
  iterators.