ds->columns = xnrealloc (ds->columns,
ds->n_columns + 1, sizeof *ds->columns);
insert_element (ds->columns, ds->n_columns, sizeof *ds->columns, before);
ds->columns = xnrealloc (ds->columns,
ds->n_columns + 1, sizeof *ds->columns);
insert_element (ds->columns, ds->n_columns, sizeof *ds->columns, before);