@Override
 public Iterator<TaskStateChange> iterateContentChangesSince(
     FileCollectionSnapshot oldSnapshot, String fileType) {
   final OutputFilesSnapshot other = (OutputFilesSnapshot) oldSnapshot;
   final Iterator<TaskStateChange> rootFileIdIterator =
       iterateRootFileIdChanges(other, fileType);
   final Iterator<TaskStateChange> fileIterator =
       filesSnapshot.iterateContentChangesSince(other.filesSnapshot, fileType);
   return Iterators.concat(rootFileIdIterator, fileIterator);
 }