code: private static RecurrenceIterator join(List<RecurrenceIterator> iterators) {
  		if (iterators.size() == 1) {
  			return iterators.get(0);
  		}
  		RecurrenceIterator first = iterators.get(0);
  		List<RecurrenceIterator> rest = iterators.subList(1, iterators.size());
  		return RecurrenceIteratorFactory.join(first, rest.toArray(new RecurrenceIterator[0]));
  	}
docs: Creates a single {@link RecurrenceIterator} that is a union of the given
  iterators.
  @param iterators the iterators
  @return the union of the given iterators
  @see RecurrenceIteratorFactory#join