Yes, hat->winter-bash synonymization makes perfect sense. Speaking of synonyms, I came across the MO thread Potential “technical” obstacle to migrations, and suggestion for a solution...

where the concern was voiced about MO->MSE migration being rejected because of tag incompatibility. MO has about 30 two-letter-prefixed tags, which MSE does not.

The fact that unprefixed synonym exists on MO is not a factor; tag synonyms work one way, from "subordinate" to "master". (Unlike synonyms in human speech). quid's attempt to change this does not appear to be going anywhere.

So I think this needs to be fixed on the MSE side, as Kaveh suggested: create synonyms mapping gr.group-theory -> group-theory, and so on; at least for the prefixed tags that are likely to exist on migrated questions. (It's not likely that kt.k-theory questions will get migrated from MO to MSE.)

@PostNoBills I am surprised to see the claim that a question cannot be migrated if the tag does not exist.

For example this question was migrated from MO and it was tagged as untagged after the migration.

Originally on MO the only tag that question had was (pr.probability).

I suppose that most of the questions shown in search for untagged site:math.stackexchange.com come from migration.

BTW thanks for stopping by in this chatroom. (Although I do not mind speaking to myself, if there is even a tiny chance that someone will read what I wrote and that it might potentially be useful.)

@MartinSleziak The policies are different for moderator migration (by Morrison, in your example) and user-driven migration (by 5 users, as in quid's example). The latter is subject to more restrictions. This is also why the moderators tend to respond to this concern with surprise: "I migrated lots of questions and never had such problems". :)

I see. I did not know about that.

I find that really strange thing to require when migrating questions.

I see that there is a feature request on meta.SO posted by quid: Drop or change automatic migration-rejection based on non-existing tags

Yes, that's the one about which I said that it "does not appear to be going anywhere". My guess is that SO has enough clueless migrators that such restrictions make sense there; and then they are applied automatically to the rest.

I guess that if this is not going to be solved by our SO overlords; the best people to handle this should be the mods of MO and MSE.

By the way, apart from the issue of rejections, there is a another reason to add synonyms: less work retagging migrated questions. In your example, Willie Wong had to add "probability" tag; with a synonym, pr.probability would be mapped to it automatically. (Then the users following the probability tag would be able to see if anything else needs to be done with the question.)

I agree with your assessment.

But I would definitely prefer if the restriction for migration were removed.

I see, if there is enough migrations, the synonyms could save some work.