code: public static RecurrenceIterator join(RecurrenceIterator first, RecurrenceIterator... rest) {
  		List<RecurrenceIterator> all = new ArrayList<RecurrenceIterator>();
  		all.add(first);
  		all.addAll(Arrays.asList(rest));
  		return new CompoundIteratorImpl(all, Collections.<RecurrenceIterator> emptyList());
  	}
docs: Generates a recurrence iterator that iterates over the union of the given
  recurrence iterators.
  @param first the first recurrence iterator
  @param rest the other recurrence iterators
  @return the union iterator