+ Grob *me = unsmob_grob (smob);
+ Real t = me->layout ()->get_dimension (ly_symbol2scm ("line-thickness"));
+ t *= robust_scm2double (me->get_property ("thickness"), 1.0);
+
+ SCM line_positions = me->get_property ("line-positions");
+
+ Interval y_ext;
+ Real space = staff_space (me);
+ if (scm_is_pair (line_positions))
+ {
+ for (SCM s = line_positions; scm_is_pair (s);
+ s = scm_cdr (s))
+ y_ext.add_point (scm_to_double (scm_car (s)) * 0.5 * space);
+ }
+ else
+ {
+ int l = Staff_symbol::line_count (me);
+ Real height = (l - 1) * staff_space (me) / 2;
+ y_ext = Interval (-height, height);
+ }
+ y_ext.widen (t/2);
+ return ly_interval2scm (y_ext);