code: func (el *CallbackEventListener) RegAttachingToTangle(f SignalEventCallback) {
  	el.ids = append(el.ids, el.em.RegisterListener(func(data interface{}) {
  		f()
  	}, event.EventAttachingToTangle))
  }
docs: // RegAttachingToTangle registers this listener to listen to when Proof-of-Work is executed.