code: func (pn *notificationRequest) validate() error {
  	if 0 == len(pn.Interests) || len(pn.Interests) > 10 {
  		return errors.New("Interests must contain between 1 and 10 interests")
  	}
  
  	if pn.APNS == nil && pn.GCM == nil && pn.FCM == nil {
  		return errors.New("PushNotification must contain a GCM, FCM or APNS payload")
  	}
  
  	return nil
  }
docs: // validate checks the notificationRequest has 0<Interests<11 and has a
  // APNS, GCM or FCM payload