public Task <Delivery> GetAsync(Guid id) => Deliveries.Find(d => d.Id == id).SingleOrDefaultAsync();
 public async Task <bool> IsParcelInDeliveryAsync(Guid parcelId)
 => await Deliveries.Find(d => d.Parcels.Any(p => p.Id == parcelId && !p.IsAvailable))
 .CountDocumentsAsync() > 0;