Was a bit worried that my answer was quite complex, with multiple maps, transposing, tracking seen values, and nlogn filtering, but it seems that’s what everyone else did too.

I need to focus on work for the next couple of weeks, so I’ll only be looking at the remaining problems on days off from now on.

Enum.zip to transpose. Wish I’d thought of that.

Also: I thought -- was much nicer than MapSet.difference. I was willing to accept a performance hit for the increased simplicity, but apparent it actually performs better since OTP 22:

As of Erlang/OTP 22, this operation is significantly faster even if both lists are very long, and using --/2 is usually faster and uses less memory than using the MapSet-based alternative mentioned above.