code: func newWriter(c Config, w io.Writer) (*gzip.Writer, error) {
  	if c.Level >= gzip.BestSpeed && c.Level <= gzip.BestCompression {
  		return gzip.NewWriterLevel(w, c.Level)
  	}
  	return gzip.NewWriter(w), nil
  }
docs: // newWriter create a new Gzip Writer based on the compression level.
  // If the level is valid (i.e. between 1 and 9), it uses the level.
  // Otherwise, it uses default compression level.