@SmokeDetector Someone upvoted that.

This question has, in my opinion, quite useful answers: Sequence with all rationals as limit points.

So I think it is worth keeping around.

There was discussion on meta whether current quality standards (closing for missing context) should be applied retroactively or not.

In any case, I have at least added definition of limit point. Are there some other suggestions what to do in order to improve this question/add context?