[dune-fem] Preconditioning CGInverseOperator

[Agnese, Marco]

Hi,
I have a question about preconditioning a Dune::Fem::Solver::CGInverseOperator. The matrix of the system is not assembled. 
I would like to build and use a preconditioner which is not assembled. Is it possible?
Because, reading the documentation, it seems that the preconditioner must be assembled and diagonal. Am I wrong?

Moreover, if I wanted to use an OEM solver in the same setting (no assembled operator and no assembled preconditioner), would it be possible to do that?
Cheers,
Marco.