code: func TrustPEM(metadata map[string]string) ([]*x509.Certificate, error) {
  	sourceFile, ok := metadata["source"]
  	if !ok {
  		return nil, errors.New("transport: PEM source requires a source file")
  	}
  
  	in, err := ioutil.ReadFile(sourceFile)
  	if err != nil {
  		return nil, err
  	}
  
  	return helpers.ParseCertificatesPEM(in)
  }
docs: // TrustPEM takes a source file containing one or more certificates
  // and adds them to the trust store.