code: func (c *Client) SetRootCAs(file string) error {
  	pool := x509.NewCertPool()
  
  	for _, name := range filepath.SplitList(file) {
  		pem, err := ioutil.ReadFile(filepath.Clean(name))
  		if err != nil {
  			return err
  		}
  
  		if ok := pool.AppendCertsFromPEM(pem); !ok {
  			return errInvalidCACertificate{
  				File: name,
  			}
  		}
  	}
  
  	c.t.TLSClientConfig.RootCAs = pool
  
  	return nil
  }
docs: // SetRootCAs defines the set of root certificate authorities
  // that clients use when verifying server certificates.
  // By default TLS uses the host's root CA set.
  //
  // See: http.Client.Transport.TLSClientConfig.RootCAs