+ find_all_sources_across_snapshots (sources_used_by_all_snapshots, true);
+
+ /* Although the region factory has a list of all regions ever created
+ * for this session, we're only interested in regions actually in
+ * playlists right now. So merge all playlist regions lists together.
+ *
+ * This will include the playlists used within compound regions.
+ */
+
+ playlists->foreach (boost::bind (merge_all_sources, _1, &sources_used_by_this_snapshot));