In this case, I just declined one of your flags because it seemed that you were flagging an answer that was wrong, on a technical level, but still an answer... As for what you really meant, I understand better, now that you've explained it and I've read your flag reason a few more times...
This is tricky - if the OP has already mentioned that Y doesn't work beforehand, as in, before the answer was written, then I say, yes, in most cases, delete it. But if the OP later edits his question and says that one of the answers doesn't work, then the answer stays.
But this depends on different perspectives. Some people feel one way or another. I personally hate to delete it, after he has put work into answering, even if the OP has already dismissed it, as it may have value to future visitors who want to try setx in this situation. This is actually a very common problem on SU, I've seen a lot of answers where someone answered even after the OP said it wouldn't work - I've even posted things without fully reading the question...In my case, I usually delete it, but sometimes I leave it, because it has value.
Thanks for bringing this up...
(Question [in question]: equivalent "echo on" for linux?)