Is it possible to set PCD preconditioner within schurPreconditioner

General Discussion
3

Actually no point :joy:, I just wonder if it’s possible to set an approximate schur complement (take PCD as an example) without the block structure .

Sorry to disturb you again, I learnt from this post Resolvent operator with FF/PETSc (MatMatSolve?) - #13 by cmd that we can set a Mat from a function, e.g.

    func real[int] PCD(real[int]& in) {
        real[int] out;
        KSPSolve(dAp, in, out);
        MatMult(dFp, out, in);
        KSPSolve(dMp, in, out);
        out *= -1.0;
        return out;
    } 
    Mat S(dAp,PCD);

But the input of schurPreconditioner should be a matrix format, I wonder if it is possible to convert from Mat to matrix?