[dune-pdelab] Sense of extracting MatrixBackend from VectorBackend

[Markus Blatt]

Hi,

while fighting the bitrot of my sample application, I decided to
rewrite it using overlappingsinglephaseflow.cc as a template.

To my great surprise the matrixbackend gets extracted from the
vectorbackend in this example, like this:

typedef Dune::PDELab::ISTLVectorBackend<1> VBE;
typedef typename VBE::MatrixBackend MatrixBackend;

I was not aware that this is possible and am still not sure whether it
makes sense in all cases.

It assumes that one always uses the same blocksize in VectorBackend
of the range and of the domain. Aren't there cases where the function
spaces of the domain and range differ and one might want to use
different block sizes?

Cheers,

Markus
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