-But aside from that, Tags are used for managing those relationships, too.
-
-Every ``Book`` has a matching `l-tag`. It's a ``Tag`` of category
-``book`` and matching slug with an added 'l-' prefix (the prefix
-is superfluous and we should remove it as some point, as it was only
-needed when tag slugs had to be unique).
-
-The `l-tag` of a ``Book`` is used on:
-
-* all of the book's fragments,
-* all of the book's descendants,
-* all of the book's descendants' fragments.
-
-This is used for:
-
-* finding fragments of a given theme in books with a given user label,
-* on a filtered book list (i.e., author's page), for eliminating
- descendants, if an ancestor is already on the list,
-* when calculating tag book counts, for eliminating descendants as above,
+Additionally, every ``Book`` has a many-to-many relationship `ancestor`,
+onnecting it to all its ancestors, with reverse relationship called
+`descendant`. This relationship is rebuilt after a `Book` is published.