vector_sort (elements, staff_priority_less);
Grob *x_common = common_refpoint_of_array (elements, me, X_AXIS);
Grob *y_common = common_refpoint_of_array (elements, me, Y_AXIS);
vector_sort (elements, staff_priority_less);
Grob *x_common = common_refpoint_of_array (elements, me, X_AXIS);
Grob *y_common = common_refpoint_of_array (elements, me, Y_AXIS);