[firedrake] Interpolate function only on the boundary

Dear all, I am wanting to interpolate an expression involving the FacetNormal, but when I execute the following code: V = FunctionSpace(mesh,'CG',1) nu = FacetNormal(mesh) g = interpolate(dot(nu,nu),V) Of course I get an AssertionError because the FacetNormal is only defined on the exterior facets of the mesh, and not the interior. My question is, is there perhaps a way to make "g" to be defined as "dot(nu,nu)" on the boundary, and 0 everywhere else? I bet this would work well. Or maybe there is a way to define "g" only on the boundary? Andrew Hicks