code: func sendAlerts(s sender, externalURL string) rules.NotifyFunc {
  	return func(ctx context.Context, expr string, alerts ...*rules.Alert) {
  		var res []*notifier.Alert
  
  		for _, alert := range alerts {
  			a := &notifier.Alert{
  				StartsAt:     alert.FiredAt,
  				Labels:       alert.Labels,
  				Annotations:  alert.Annotations,
  				GeneratorURL: externalURL + strutil.TableLinkForExpression(expr),
  			}
  			if !alert.ResolvedAt.IsZero() {
  				a.EndsAt = alert.ResolvedAt
  			} else {
  				a.EndsAt = alert.ValidUntil
  			}
  			res = append(res, a)
  		}
  
  		if len(alerts) > 0 {
  			s.Send(res...)
  		}
  	}
  }
docs: // sendAlerts implements the rules.NotifyFunc for a Notifier.