code: func (a *AuthenticateUserRequest) CheckAndSetDefaults() error {
  	if a.Username == "" {
  		return trace.BadParameter("missing parameter 'username'")
  	}
  	if a.Pass == nil && a.U2F == nil && a.OTP == nil && a.Session == nil {
  		return trace.BadParameter("at least one authentication method is required")
  	}
  	return nil
  }
docs: // CheckAndSetDefaults checks and sets defaults