@Nullable
  private MergeChunk getNextChunk() {
    int start = getNextChunkStart();
    int size = 0;

    if (isInBounds(start)) {
      size = myGroupSequentialChangeLists ? getGroupSize(start) : 1;
    }

    return size > 0 ? new MergeChunk(start, size) : null;
  }
@Nullable
  private MergeChunk getNextChunk()