-void gis_viewer_add(GisViewer *self, GisObject *object);
-void gis_viewer_remove(GisViewer *self, GisObject *object);
+gpointer gis_viewer_add(GisViewer *self, GisObject *object,
+ gint level, gboolean sort);
+void gis_viewer_remove(GisViewer *self, gpointer ref);