public void On(DocumentFormatHasBeenDeleted e)
 {
     _documents.FindAndModify(e, (DocumentDescriptorId)e.AggregateId, d =>
     {
         d.RemoveFormat(e.DocumentFormat);
     });
 }
 private void When(DocumentFormatHasBeenDeleted e)
 {
     Formats.Remove(e.DocumentFormat);
 }