How can I use Block Forms, Multiphenicsx, and Lagrange Multipliers to Solve Poisson's Equation with Pure Neumann Boundary Conditions?

The final implementation, that will resemble the one in the post I linked above, will be different from the old tutorial.

In particular

R = dolfinx.fem.FunctionSpace(domain, ("Lagrange", 2))

is not an equivalent of the former

R = FunctionSpace(mesh, "R", 0)

(i.e., a FE space with a single global degree of freedom). The former "R" space has not been reimplemented yet in dolfinx, and that is one of the reasons why the new implementation will need to be different.

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