Quick-fix for Guido's bug attached. I'm not familiar with this part of the code, yet, so take this tentatively. I just changed

    while (containers > 1) {

to

    while (containers) {

---

@Guido

My comments were assuming `f(**x for x in y)` meant `f({**x for x in y})`.

I see your reasoning, but I don't like how your version has

     (x for y in z for x in y) ==  (*y for y in z)
    f(x for y in z for x in y) != f(*y for y in z)

This seems like a tripping point. I've never wanted to unpack a 2D iterable into an argument list, so personally I'm not convinced by the value-add either.