σ⁡(u+v)⁢σ⁡(u-v)⁢σ⁡(x+y)⁢σ⁡(x-y)+σ⁡(v+x)⁢σ⁡(v-x)⁢σ⁡(u+y)⁢σ⁡(u-y)+σ⁡(x+u)⁢σ⁡(x-u)⁢σ⁡(v+y)⁢σ⁡(v-y)=0.