code: func (o *httpOps) DownloadSignature(a *asc) (readSeekCloser, bool, error) {
  	ensureLogger(o.Debug)
  	diag.Printf("downloading signature from %v", a.Location)
  	ascFile, err := a.Get()
  	if err == nil {
  		return ascFile, false, nil
  	}
  	if _, ok := err.(*statusAcceptedError); ok {
  		log.Printf("server requested deferring the signature download")
  		return NopReadSeekCloser(nil), true, nil
  	}
  	return nil, false, errwrap.Wrap(errors.New("error downloading the signature file"), err)
  }
docs: // DownloadSignature takes an asc instance and tries to get the
  // signature. If the remote server asked to to defer the download,
  // this function will return true and no error and no file.