docs: def proportion_merge(brands, exemplars):
      """ Return the proportion of a brand's followers who also follower an
      exemplar. We merge all exemplar followers into one big pseudo-account."""
      scores = {}
      exemplar_followers = set()
      for followers in exemplars.values():
          exemplar_followers |= followers
  
      for brand, followers in brands:
          scores[brand] = _proportion(followers, exemplar_followers)
      return scores