sum 0 0 sum s x + sum x s x + x 0 x + x s y s + x y 2.2 sum s x sum x + x s y + x y sum s x + sum x s x true sum s x sum x true s 1 1 1 sum 1 0 0 0 0 + 2 0 sum 1 1 0 + 2 0 sum s x + sum x s x false + x s y + x y true s 1 1 1 sum 1 0 0 0 0 + 2 0 sum 1 0 + 2 2 0 NaTT certifiable-1.6