code: func (req *GenerateServerKeysRequest) CheckAndSetDefaults() error {
  	if req.HostID == "" {
  		return trace.BadParameter("missing parameter HostID")
  	}
  	if len(req.Roles) != 1 {
  		return trace.BadParameter("expected only one system role, got %v", len(req.Roles))
  	}
  	return nil
  }
docs: // CheckAndSetDefaults checks and sets default values