docs: def get_epno(self, episode: Episode):
          """Return epno for an Episode instance.
  
          epno is a string formatted with the episode number and type, e.g., S1,
          T2.
  
          >>> x = EpisodeTypes([EpisodeType(1, 'foo', 'F')])
          >>> ep = Episode(type=1, number=2)
          >>> x.get_epno(ep)
          'F2'
  
          """
          return '{}{}'.format(self[episode.type].prefix, episode.number)