The is a backend for Eigen3 implemented and for a minimal own data-structure. This needs to be extended for more libraries, e.g. MTL4 and PETSc. For PETSc already a basic implementation is available (for the old dec-pde library) that probably can easily be adapted.
There are currently just a few requirements for the backend. Basically it must be possible to run the CG or BCGS method with the implemented backend on the iterative solvers in
The backend interface should be documented somewhere.