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Here's what I was thinking. We really need to successfully execute someone this round. One of the pair, Nera/Zubr, is Mafia, but even if we're lucky enough to guess correctly, we still won't be able to execute them because the Vigilante gets a pass the next day. We need to look for the Serial Killer, and Nera or Zubr can be exposed at night – either with a serum from the Doctor or I can investigate them. It's obvious that they're from opposing factions.