   created timestamp not null default current_timestamp,
   modified timestamp not null default current_timestamp,
   entity_id uuid not null references entity (entity_id),  # TODO enforce kind
-  previous_id uuid null references unit (version_id),
+  previous_id uuid null references unit_version (version_id),
   language varchar(5) not null default 'en'
     constraint lang_check check (language ~* '^\w{2}(-\w{2})?$'),
   name text not null,

  created timestamp not null default current_timestamp,
  modified timestamp not null default current_timestamp,
  entity_id uuid not null references entity (entity_id), # TODO enforce kind
  previous_id uuid null references subject (version_id),
  language varchar(5)    not null default 'en'
    constraint lang_check check (language ~* '^\w{2}(-\w{2})?$'),
  name text not null,

  created timestamp not null default current_timestamp,
  modified timestamp not null default current_timestamp,
  entity_id uuid not null references entity (entity_id), # TODO enforce kind
  previous_id uuid null references card_version (version_id),
  language varchar(5) not null default 'en'
    constraint lang_check check (language ~* '^\w{2}(-\w{2})?$'),
  name text not null,
