Grob *common_x = common_refpoint_of_array (elements, me, X_AXIS);
Interval xext = Axis_group_interface::relative_group_extent (elements, common_x, X_AXIS);
Grob *common_x = common_refpoint_of_array (elements, me, X_AXIS);
Interval xext = Axis_group_interface::relative_group_extent (elements, common_x, X_AXIS);