Good point; I was thinking of cw0f{(w5w1)w2}. cw0{f(w5w1)f(w2)}, or
equivalently cw0{f(w5w1)fw2}, should work if (w5w1) is smaller than
w2. The mirror device will limit both devices to the size of the
smaller one, so if w2 is smaller than (w5w1), f(w5w1) will be
truncated in the mirror {f(w5w1)fw2}.
There's still the question of how to initialise the second device in
the mirror-to-be. I would have copyworm-ed f(w5w1) to f(w2). I'm
nervous that copying from cw0f(w5w1) may not yield exactly the same
bytes as copying from f(w5w1).
