node = pool_malloc (a->pool, sizeof *node);
tower_insert (&a->unit_tower, n_units, &node->unit_node, NULL);
tower_insert (&a->pixel_tower, (size + a->padding) * n_units,
&node->pixel_node, NULL);
node = pool_malloc (a->pool, sizeof *node);
tower_insert (&a->unit_tower, n_units, &node->unit_node, NULL);
tower_insert (&a->pixel_tower, (size + a->padding) * n_units,
&node->pixel_node, NULL);