code: def iter_extensions(prefix, event_id):
      
      extension = '{prefix}{event_id}'.format(prefix=prefix, event_id=event_id)
      yield extension
      suffix = 1
      while True:
          yield '{extension}{suffix}'.format(extension=extension, suffix=suffix)
          suffix += 1
docs: Return extension (prefix + event_id) with an optional suffix which is
         incremented step by step in case of collision