[petsc-dev] Is xout always already zero'ed when being called back on PCShellSetApply ?

[Franck Houssen]

When using http://www.mcs.anl.gov/petsc/petsc-current/docs/manualpages/PC/PCShellSetApply.html, do I have the guaranty that xout from " PetscErrorCode apply ( PC pc, Vec xin, Vec xout)" has always been previously filled with zeros ? 
I may have to fill xout by blocks (I would += several possibly overlapping blocks: I need to make sure xout is first filled with zero to get the correct result). 

Franck 
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