code: func (el *CallbackEventListener) RegSentTransfers(f BundleEventCallback) {
  	el.ids = append(el.ids, el.em.RegisterListener(func(data interface{}) {
  		f(data.(bundle.Bundle))
  	}, event.EventSentTransfer))
  }
docs: // RegSentTransfers registers the given callback to execute when a transfer is sent off.