用xtabond2做GMM出现以下结果，请懂的大神教教我，这是什么意思，怎么处理？拜谢各位大神！


. xtabond2 CO2percapit  tadr exgdp urbanrate IFDIGDP,gmm( tadr exgdp urbanrate IFDIGDP,eq(diff) lag(2,2)) iv( y92 y93 y94
y95 y96 y97 y98 y99 y00 y01 y02 y03 y04 y05 y06 y07 y08 y09)
Favoring speed over space. To switch, type or click on mata: mata set matafavor space, perm.
Laglimits(2,2) must have two arguments.
r(198);