Assembly of local Mat's to a parallel Mat formally defined by A = \sum_i Ri^T Ai Ri.
My Ai's are originally defined in sub-communicators. In Petsc, I was told before there are no counterpart of Ri that can promote the mat Ai to sup-communicator where A stays. (VecScatter can only acts on Vec but not Mat) So, what is the easiest way to achieve this task? Note that Ai's are already defined in sub-communicators.
