code: func CertPoolFromCertAuthorities(cas []CertAuthority) (*x509.CertPool, error) {
  	certPool := x509.NewCertPool()
  	for _, ca := range cas {
  		keyPairs := ca.GetTLSKeyPairs()
  		if len(keyPairs) == 0 {
  			continue
  		}
  		for _, keyPair := range keyPairs {
  			cert, err := tlsca.ParseCertificatePEM(keyPair.Cert)
  			if err != nil {
  				return nil, trace.Wrap(err)
  			}
  			certPool.AddCert(cert)
  		}
  		return certPool, nil
  	}
  	return certPool, nil
  }
docs: // CertPoolFromCertAuthorities returns certificate pools from TLS certificates
  // set up in the certificate authorities list