code: public static RecurrenceIterator except(
              RecurrenceIterator included, RecurrenceIterator excluded) {
          return new CompoundIteratorImpl(
                  Collections.singleton(included),
                  Collections.singleton(excluded));
      }
docs: an iterator over all the dates included except those excluded, i.e.
  <code>inclusions - exclusions</code>.
  Exclusions trump inclusions, and {@link DateValue dates} and
  {@link DateTimeValue date-times} never match one another.
  @param included non null.
  @param excluded non null.
  @return non null.