[firedrake] R space

Dear firedrakers, Can we have a discussion about the "R" space (function space of global constants), which if I understand correctly has not been implemented? Clearly, the R space is not good for performance, since it inserts an entire column or row into the matrix of non-zero entries, and the high performance solution is to solve the problem with a Krylov solver by projecting out the constant from the solver as Lawrence has recently implemented. However, the R space is useful for verifying other parts of Firedrake by e.g. computing eigenvalues of the Laplacian etc. I argue that we want this feature for testing, even though we don't expect to use it for big production code. --cjc