code: func (m *Messenger) classify(info MessageInfo, e Entry) Action {
  	if info.Message != nil {
  		return TextAction
  	} else if info.Delivery != nil {
  		return DeliveryAction
  	} else if info.Read != nil {
  		return ReadAction
  	} else if info.PostBack != nil {
  		return PostBackAction
  	} else if info.OptIn != nil {
  		return OptInAction
  	} else if info.ReferralMessage != nil {
  		return ReferralAction
  	} else if info.AccountLinking != nil {
  		return AccountLinkingAction
  	}
  	return UnknownAction
  }
docs: // classify determines what type of message a webhook event is.