+ virtual framecnt_t length (framepos_t pos) const = 0;
+ virtual void update_length (framepos_t pos, framecnt_t cnt) = 0;
+
+ virtual const Glib::ustring& path() const = 0;
+
+ virtual framepos_t natural_position() const { return 0; }
+
+ void mark_for_remove();
+
+ virtual void mark_streaming_write_started () {}
+ virtual void mark_streaming_write_completed () = 0;
+
+ virtual void session_saved() {}
+