Currently the class VERSION_REPOSITORY<T> does not
include a foreign key attribute representing the id of the
repository to which it belongs. Where objects of this class are
logically part of a distinct repository such as a patient EHR,
not having such an id may be dangerous if the database is
corrupted, or if whole VERSION_REPOSITORY instances are sent to
another location. It is also inconvenient when querying for
VERSION_REPOSITORY instances not to have the foreign key of the
owning repository.