In the case of new articles, the Tree's existing filter is applied to them and new articles that survive the filter are threaded into the tree.
In the case of deleted articles, their children are reparented to the youngest surviving ancestor and then the Article objects are deleted.
A Diffs object summarizes these changes to make it easier for clients to synchronize their Views to the changes.
|typedef std::list< Added >||added_t|
|typedef std::list< Reparent >||reparented_t|