I just noticed that an user, call them $A$ (for "answer", not their real user name) has edited this problem, apparently with the aim of adding an extra hypothesis (namely that the measure of the derived set of the $r_n$ is $0$) that would "make" the incorrect answer by $A$ "correct". (I had left earlier comments to $A$'s answer pointing out the error.) This is a form of cheating in my opinion and should not be tolerated. I cannot quite understand the motive and if $A$ understands that this should be a big NO! NO!
Hope this could be addressed, sorry if it had already been discussed, didn't see something similar in list of questions already posted, when I typed the title of my question.
Are there any general policy, norms, actions by moderators that could prevent something like this happening. I assume that users should themselves follow an honor code, but when that does not seem to work is there anything that could/should be done? (P.S. I had also posted an answer to that problem, but deleted it, as it was incomplete. There is yet another answer there, by another user.) (P.P.S. I wish there was a tag called cheating.)