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Preconditioner routines

The base PSBLAS library contains the implementation of two simple preconditioning techniques:

- Diagonal Scaling
- Block Jacobi with ILU(0) factorization

The supporting data type and subroutine interfaces are defined in the module `psb_prec_mod`. The old interfaces `psb_precinit` and `psb_precbld` are still supported for backward compatibility

Subsections

- init -- Initialize a preconditioner
- build -- Builds a preconditioner
- apply -- Preconditioner application routine
- descr -- Prints a description of current preconditioner
- clone -- clone current preconditioner
- free -- Free a preconditioner