- the linear operator to be preconditioned, usually the mass matrix.
Notes
The operator can be either passed in a) monolithic global ordering, b) pressure-only global ordering
or c) interface pressure ordering (if -ksp_fetidp_pressure_all false).
In cases b) and c), the pressure ordering of dofs needs to satisfy
pid_1 < pid_2 iff gid_1 < gid_2
where pid_1 and pid_2 are two different pressure dof numbers and gid_1 and gid_2 the corresponding
id in the monolithic global ordering.