We likely are going to propose "no nested list comprehensions because they are hard to follow" in the coding guidelines. Everyone involved in QIIME development will of course be invited to weigh in on the guidelines, and we'll decide then what is and is not going in. If "no nested list comprehensions" makes it into the updated guidelines, you'll need to remove this one then. If you want to remove it now (while the code is fresh in your mind) that's great. If you'd rather leave it in for now, that is also OK, but if it does end up in the coding guidelines you'll have to come back and change then when the code won't be fresh in your mind anymore.