code: func ParseGeneration(filename string) int64 {
  	assert(strings.HasPrefix(filename, "_"))
  	parts := strings.Split(StripExtension(filename)[1:], "_")
  	// 4 cases:
  	// segment.ext
  	// segment_gen.ext
  	// segment_codec_suffix.ext
  	// segment_gen_codec_suffix.ext
  	if n := len(parts); n == 2 || n == 4 {
  		v, err := strconv.ParseInt(parts[1], 36, 64)
  		assert(err == nil)
  		return v
  	}
  	return 0
  }
docs: /* Returns the generation from this file name, or 0 if there is no generation. */