}
        if (upperBound != null) {
          map = map.headMap(upperBound);
        }
        return map;
      }
      return null;
    }

    @Override
    void maintainEmptyInvariant() {
      updateWholeRowField();
      if (wholeRow != null && wholeRow.isEmpty()) {
        backingMap.remove(rowKey);
        wholeRow = null;
        backingRowMap = null;
      }
    }