[firedrake] sub-matrix assembly

Hello. I would like to know if you can tell Firedrake to run the assembly over part of a mesh. For example, suppose I have a stiffness matrix (with variable coefficient) defined over a global mesh, but I want to do some domain decomposition where I solve on a sub-domain that may be changing with iteration. I don't want to assemble the entire matrix (then take a sub-matrix) because this can be inefficient. Is there a way to only assemble over a subset of elements, and solve using the corresponding sub-matrix? -Shawn