For DV_IDENTIFIER sometimes we need to specify the date interval (or only a start date or an end date) when the information is considered active (valid). Currently this is done in the archetype level by using CLUSTER and ELEMENTs, but it would be easier just to have this information on the type itself. These attributes should be optional and if not filled-in they should be assumed active (perhaps by assuming them as min-date, max-date).
0..1 DV_IDENTIFIER.validity = DV_INTERVAL<DV_DATE>
NOT DONE: use CLUSTER archetypes to add in meta-data to do with data integration.