code: public Sequence setSequence(Integer sequence) {
  		Sequence prop = (sequence == null) ? null : new Sequence(sequence);
  		setSequence(prop);
  		return prop;
  	}
docs: Sets the revision number of the journal entry. The organizer can
  increment this number every time he or she makes a significant change.
  @param sequence the sequence number
  @return the property that was created
  @see <a href="http://tools.ietf.org/html/rfc5545#page-138">RFC 5545
  p.138-9</a>
  @see <a href="http://tools.ietf.org/html/rfc2445#page-131">RFC 2445
  p.131-3</a>