code: func (l *Logger) prefixAndExt() (prefix, ext string) {
  	filename := filepath.Base(l.filename())
  	ext = filepath.Ext(filename)
  	prefix = filename[:len(filename)-len(ext)] + "-"
  	return prefix, ext
  }
docs: // prefixAndExt returns the filename part and extension part from the Logger's
  // filename.