public Task Execute(StartScan c) => repository.Execute(c.HappeningId, h => h.ValidateStartScan(c.ToScanInfo()));
 public Task Execute(UnlinkPersonFromHappening c) =>
 // Validate first, PersonMessageHandler takes care of the next step
 happeningsRepository.Execute(
     RootAggregateId,
     h => h.ValidatePersonUnlink(c.HappeningId, c.PersonId));