Zero mean constraint

[Jason]

For torsion laplace equation with neumann boundary, we may need add additional boundary (zero mean boundary) to remove singular. How could we add this additional boundary.
Torsion Laplace equation as following:

Additional boundary to remove singlarity.

Thanks
Jason

[joachim]

there is NumberSpace - a finite element space have one globally constant basisfunction. Choose your Lagrange multiplier in NumberSpace