\[\sigma\left(u+v\right)\sigma\left(u-v\right)\sigma\left(x+y\right)\sigma\left(%
x-y\right)+\sigma\left(v+x\right)\sigma\left(v-x\right)\sigma\left(u+y\right)%
\sigma\left(u-y\right)+{\sigma\left(x+u\right)\sigma\left(x-u\right)\sigma%
\left(v+y\right)\sigma\left(v-y\right)=0.}\]