+Interval
+Axis_group_interface::generic_bound_extent (Grob *me, Grob *common, Axis a)
+{
+ /* trigger the callback to do skyline-spacing on the children */
+ if (a == Y_AXIS)
+ (void) me->get_property ("vertical-skylines");
+
+ extract_grob_set (me, "elements", elts);
+ vector<Grob *> new_elts;
+
+ SCM interfaces = me->get_property ("bound-alignment-interfaces");
+
+ for (vsize i = 0; i < elts.size (); i++)
+ for (SCM l = interfaces; scm_is_pair (l); l = scm_cdr (l))
+ if (elts[i]->internal_has_interface (scm_car (l)))
+ new_elts.push_back (elts[i]);
+
+ if (!new_elts.size ())
+ return robust_relative_extent (me, common, a);
+
+ if (!common)
+ common = common_refpoint_of_array (new_elts, me, a);
+
+ return relative_maybe_bound_group_extent (new_elts, common, a, true);
+}
+