Added Optional<> around Ref<> -- TBD: combine Optional into Ref<>
Tinkering with Ref class
Moved Optional<> capabilities inside Ref<>.
Revised model element registry hierarchy.
Shook out model element registry and self references.
Improved Ref<> class.
Moved parent foreign key down in the database entity hierarchy.
Added tentative VSet versioned set class.
Refactoring in progress to make relationships full-fledged objects with attributes.
Renamed model elements to entities (anticipating that relationships become elements also).
Expanded infrastructure aimed at lazy loading.
Better documentation in top level classes.
Switched to entities w/o joins.
Revised data model
Reverted to use of DESTROYED flag for deleted entities.
Start of API with reversible actions.
Lots of refactoring & reorganizing.