25   Mat               A, S;  in main()  local
45   PetscCall(MatCreateShell(PETSC_COMM_WORLD, 2, 2, 2, 2, user, &S));  in main()
46   PetscCall(MatShellSetOperation(S, MATOP_GET_DIAGONAL, (PetscErrorCodeFn *)MatGetDiagonal_User));  in main()
47   PetscCall(MatSetUp(S));  in main()
49   PetscCall(MatShift(S, 42));  in main()
50   PetscCall(MatGetDiagonal(S, Y));  in main()
52   PetscCall(MatDiagonalSet(S, X, ADD_VALUES));  in main()
53   PetscCall(MatGetDiagonal(S, Y));  in main()
55   PetscCall(MatScale(S, 42));  in main()
56   PetscCall(MatGetDiagonal(S, Y));  in main()
60   PetscCall(MatDestroy(&S));  in main()