+ void pre_combine (std::vector<boost::shared_ptr<Region> >&);
+ void post_combine (std::vector<boost::shared_ptr<Region> >&, boost::shared_ptr<Region>);
+ void pre_uncombine (std::vector<boost::shared_ptr<Region> >&, boost::shared_ptr<Region>);
+
+private:
+ CrossfadeListProperty _crossfades;
+ Crossfades _pending_xfade_adds;