In Python you can have multiple iterators in a list comprehension, like
[(x,y) for x in a for y in b]
[x for x in a for a in b]
Gee, I guess I found the anwser: I was not taking care enough about which loop is inner and which is outer. The list comprehension should be like:
[x for b in a for x in b]
to get the desired result, and yes, one current value can be the iterator for the next loop :-). Sorry for the noise.