Hello petsc's friends I have a curiosity about a sentence in the user guide about CPR preconditioner:
*"**The Constrained Pressure Preconditioner (CPR) can be implemented using PCCOMPOSITE <https://petsc.org/release/manualpages/PC/PCCOMPOSITE/> with PCGALERKIN <https://petsc.org/release/manualpages/PC/PCGALERKIN/>. CPR first solves an * *RAP subsystem, updates the residual on all variables (PCCompositeSetType <https://petsc.org/release/manualpages/PC/PCCompositeSetType/>(pc,PC_COMPOSITE_MULTIPLICATIVE <https://petsc.org/release/manualpages/PC/PCCompositeType/>)), and then applies a simple ILU like preconditioner on all the variables.**"* I am certainly lacking some background here, would you be able to explain me a bit better how this is working? Do you have a minimal working example in the tutorials? Many thanks as always! :)
