@Nullable
    PerClientLocalUndoRedoSnapshot getUndoRedoSnapshotForDocument(DocumentReference reference) {
      CommandMerger currentMerger = myCurrentMerger;
      if (currentMerger != null && currentMerger.hasActions())
        return null;

      LocalCommandMergerSnapshot mergerSnapshot = myMerger.getSnapshot(reference);
      if (mergerSnapshot == null)
        return null;

      return new PerClientLocalUndoRedoSnapshot(
        mergerSnapshot,
        myUndoStacksHolder.getStack(reference).snapshot(),
        myRedoStacksHolder.getStack(reference).snapshot(),

        myManager.myAdjustableUndoableActionsHolder.getStack(reference).snapshot()
      );
    }
@Nullable
    PerClientLocalUndoRedoSnapshot getUndoRedoSnapshotForDocument(DocumentReference reference)