| x 1 y +z y 1 z+x z 1 x+y|
1+x+y+z
x +y +z
0
1
C1→C1+C2+C3
1+x+y+z1y+z1+x+y+z1z+x1+x+y+z1x+y
=(1+x+y+z)|11y+z11z+x11x+y|=0