Spanner *spanner = dynamic_cast<Spanner*> (me);
Grob *common = spanner->get_bound (LEFT)->common_refpoint (spanner->get_bound (RIGHT), X_AXIS);
Spanner *spanner = dynamic_cast<Spanner*> (me);
Grob *common = spanner->get_bound (LEFT)->common_refpoint (spanner->get_bound (RIGHT), X_AXIS);