static Drul_array<Skyline> skylines (Grob *me, Grob *right_col);
static Grob *extremal_break_aligned_grob (Grob *me, Direction, Direction, Interval *);
static Drul_array<Skyline> skylines (Grob *me, Grob *right_col);
static Grob *extremal_break_aligned_grob (Grob *me, Direction, Direction, Interval *);