code: func CertPoolFromFile(filename string) (*x509.CertPool, error) {
  	b, err := ioutil.ReadFile(filename)
  	if err != nil {
  		return nil, fmt.Errorf("can't read CA file: %v", filename)
  	}
  	cp := x509.NewCertPool()
  	if !cp.AppendCertsFromPEM(b) {
  		return nil, fmt.Errorf("failed to append certificates from file: %s", filename)
  	}
  	return cp, nil
  }
docs: // CertPoolFromFile returns an x509.CertPool containing the certificates
  // in the given PEM-encoded file.
  // Returns an error if the file could not be read, a certificate could not
  // be parsed, or if the file does not contain any certificates