PCL语言提取节点应力函数中参数derivation

PCL函数res_utl_extract_nodal_results (result_ids, node_list, derivation, cid, data_type, nres, ids, results, minloc, maxloc)可用于提取节点应力,
想用derivation中由stress tensor提取vector,只想看XX,YY,XY三个力,这个参数怎么取?看pcl帮助文档上参数说明:可用"XXYYXY",但调用始终不成功,望大家不吝赐教!