## PC

- registering preconditioners Solves a linear system in parallel with KSP,
  demonstrating how to register a new preconditioner (PC) type.
  Input parameters include:
  -m <mesh_x> : number of mesh points in x-direction
  -n <mesh_y> : number of mesh points in y-direction
- setting a user-defined shell preconditioner Solves a linear system in parallel with KSP. Also
  illustrates setting a user-defined shell preconditioner and using the
  Input parameters include:
  -user_defined_pc : Activate a user-defined preconditioner
- setting a user-defined shell preconditioner
  Solves a linear system in parallel with KSP. Also indicates
  use of a user-provided preconditioner. Input parameters include:
  -user_defined_pc : Activate a user-defined preconditioner