void set_bound (Direction d, Grob *);
Item *get_bound (Direction d) const;
void set_bound (Direction d, Grob *);
Item *get_bound (Direction d) const;
static bool less (Spanner *const &, Spanner *const &);
virtual Grob *find_broken_piece (System *) const;
virtual void derived_mark () const;
static bool less (Spanner *const &, Spanner *const &);
virtual Grob *find_broken_piece (System *) const;
virtual void derived_mark () const;