code: func (e *Event) Cancel(err ...error) {
  	e.canceled = true
  
  	if len(err) > 0 {
  		e.Err = err[0]
  	}
  }
docs: // Cancel can be called in before_<EVENT> or leave_<STATE> to cancel the
  // current transition before it happens. It takes an opitonal error, which will
  // overwrite e.Err if set before.