[firedrake] .dx without operand

I'm trying to express the following in UFL: http://amcg.ese.ic.ac.uk/~ctj10/images/k.png where A and B are matrices and u is a vector of solution variables. For the case of k=1 I can write e.g. -1*([1*u[0].dx(0) + 2*u[1].dx(0) + 5*u[0].dx(1) + 6*u[1].dx(1), 3*u[0].dx(0) + 4*u[1].dx(0) + 7*u[0].dx(1) + 8*u[0].dx(0)]) But how can I write this for k>1? Having e.g. (1*u[0].dx(0) + 2*u[0].dx(1) + ...)**2 will result in terms that represent (du/dx)**2 not (d**2)u/(dx**2). Is there a way of expressing .dx() (of arbitrary order) without an operand in UFL, so we can just have d^k/dx^k ?