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[Jake Ostien <tostien@xxxxxxxxx>]

Hi,

I am trying to compute some reaction forces, currently I am trying to 
assemble the stiffness matrix and compute the matrix vector product with 
the converged solution vector to get a vector of forces.  Here is what I 
am trying.

<u is the converged solution>
Matrix A;
assemble(A, *a, mesh);
Vector f;
A.mat().mult(u.vector().vec(), f.vec());

First an issue.  I am getting a seg fault coming in 
DiscreteFunction::interpolate as part of the assemble step.  I haven't 
figured out why yet, but I'm looking.

Next, a few questions.
Is there an easier (or cleaner) way to accomplish this?
I have a mixed formulation.  Is is possible to only assemble the 
stiffness matrix for a SubSystem? 

Thanks,
Jake