+ static Real minimum_distance (Grob *me, Grob *right_col);
+ static vector<Item*> right_note_columns (Grob *me);
+ static vector<Item*> left_note_columns (Grob *me);
+ static Item* right_column (Grob *me);
+ static Item* left_column (Grob *me);
+ static Drul_array<Skyline> skylines (Grob *me, Grob *right_col);
+
+ DECLARE_GROB_INTERFACE();