tower_insert (&a->unit_tower, n_units, &node->unit_node, NULL);
tower_insert (&a->pixel_tower, size * n_units, &node->pixel_node, NULL);
tower_insert (&a->unit_tower, n_units, &node->unit_node, NULL);
tower_insert (&a->pixel_tower, size * n_units, &node->pixel_node, NULL);