ns_id_cmp
return ns_id_cmp(node_to_ns(a)->ns_id, node_to_ns(b)->ns_id);
return ns_id_cmp(node_to_ns_unified(a)->ns_id, node_to_ns_unified(b)->ns_id);
return ns_id_cmp(node_to_ns_owner(a)->ns_id, node_to_ns_owner(b)->ns_id);