Preconditioner Application

  The setup may differ per preconditioner, but all methods are applied in the same manner:

 PC P;        /* created as described above */
 Vec rhs,sol; /* declaration of vectors */
 [... lots of code ...]
 PCApply(P,rhs,sol);

(The Vec type is a Petsc datastructure; the conversion of user structures to such a type is explained in section 3.)