code: func (ca *CertAuthorityV2) FirstSigningKey() ([]byte, error) {
  	if len(ca.Spec.SigningKeys) == 0 {
  		return nil, trace.NotFound("%v has no signing keys", ca.Metadata.Name)
  	}
  	return ca.Spec.SigningKeys[0], nil
  }
docs: // FirstSigningKey returns first signing key or returns error if it's not here