a x1 b b x1 b b b x1 a x1 2.2 a x1 b b x1 a x1 b x1 b b b x1 a x1 true b b b x1 a x1 a x1 b x1 a x1 b b x1 true a 1 1 4 b 1 1 2 a 1 1 3 b 1 1 0 a x1 b b x1 b b b x1 a x1 NaTT certifiable-1.6