Add sanity check for the positive branch of sigma, too

In addition to checking

|a+b|^2 = |a|^2 + 2 a.b + b^2 >= 0

one can also check

|a-b|^2 = |a|^2 - 2 a.b + b^2 >= 0

This gives limits to sigma[1] = a.b as -0.5*(|a|^2 + |b|^2) <= a.b <= 0.5*(|a|^2 + |b|^2).